Most model attribute types try to cast a given value before serializing
it.  This allows uncast values to be passed to finder methods and still
be serialized appropriately.  However, when persisting a model, this
cast is unnecessary because the value will already have been cast by
`ActiveModel::Attribute#value`.

To eliminate the overhead of a 2nd cast, this commit introduces a
`ActiveModel::Type::SerializeCastValue` module.  Types can include this
module, and their `serialize_cast_value` method will be called instead
of `serialize` when serializing an already-cast value.

To preserve existing behavior of any user types that subclass Rails'
types, `serialize_after_cast` will only be called if the type itself
(not a superclass) includes `ActiveModel::Type::SerializeCastValue`.
This also applies to type decorators implemented via `DelegateClass`.

Benchmark script:

  ```ruby
  require "active_model"
  require "benchmark/ips"

  class ActiveModel::Attribute
    alias baseline_value_for_database value_for_database
  end

  VALUES = {
    my_big_integer: "123456",
    my_boolean: "true",
    my_date: "1999-12-31",
    my_datetime: "1999-12-31 12:34:56 UTC",
    my_decimal: "123.456",
    my_float: "123.456",
    my_immutable_string: "abcdef",
    my_integer: "123456",
    my_string: "abcdef",
    my_time: "1999-12-31T12:34:56.789-10:00",
  }

  TYPES = VALUES.to_h { |name, value| [name, name.to_s.delete_prefix("my_").to_sym] }

  class MyModel
    include ActiveModel::API
    include ActiveModel::Attributes

    TYPES.each do |name, type|
      attribute name, type
    end
  end

  TYPES.each do |name, type|
    $attribute_set ||= MyModel.new(VALUES).instance_variable_get(:@attributes)
    attribute = $attribute_set[name.to_s]

    puts "=" * 72
    Benchmark.ips do |x|
      x.report("#{type} before") { attribute.baseline_value_for_database }
      x.report("#{type} after") { attribute.value_for_database }
      x.compare!
    end
  end
  ```

Benchmark results:

  ```
  ========================================================================
  Warming up --------------------------------------
    big_integer before   100.417k i/100ms
     big_integer after   260.375k i/100ms
  Calculating -------------------------------------
    big_integer before      1.005M (± 1.0%) i/s -      5.121M in   5.096498s
     big_integer after      2.630M (± 1.0%) i/s -     13.279M in   5.050387s

  Comparison:
     big_integer after:  2629583.6 i/s
    big_integer before:  1004961.2 i/s - 2.62x  (± 0.00) slower

  ========================================================================
  Warming up --------------------------------------
        boolean before   230.663k i/100ms
         boolean after   299.262k i/100ms
  Calculating -------------------------------------
        boolean before      2.313M (± 0.7%) i/s -     11.764M in   5.085925s
         boolean after      3.037M (± 0.6%) i/s -     15.262M in   5.026280s

  Comparison:
         boolean after:  3036640.8 i/s
        boolean before:  2313127.8 i/s - 1.31x  (± 0.00) slower

  ========================================================================
  Warming up --------------------------------------
           date before   148.821k i/100ms
            date after   298.939k i/100ms
  Calculating -------------------------------------
           date before      1.486M (± 0.6%) i/s -      7.441M in   5.006091s
            date after      2.963M (± 0.8%) i/s -     14.947M in   5.045651s

  Comparison:
            date after:  2962535.3 i/s
           date before:  1486459.4 i/s - 1.99x  (± 0.00) slower

  ========================================================================
  Warming up --------------------------------------
       datetime before    92.818k i/100ms
        datetime after   136.710k i/100ms
  Calculating -------------------------------------
       datetime before    920.236k (± 0.6%) i/s -      4.641M in   5.043355s
        datetime after      1.366M (± 0.8%) i/s -      6.836M in   5.003307s

  Comparison:
        datetime after:  1366294.1 i/s
       datetime before:   920236.1 i/s - 1.48x  (± 0.00) slower

  ========================================================================
  Warming up --------------------------------------
        decimal before    50.194k i/100ms
         decimal after   298.674k i/100ms
  Calculating -------------------------------------
        decimal before    494.141k (± 1.4%) i/s -      2.510M in   5.079995s
         decimal after      3.015M (± 1.0%) i/s -     15.232M in   5.052929s

  Comparison:
         decimal after:  3014901.3 i/s
        decimal before:   494141.2 i/s - 6.10x  (± 0.00) slower

  ========================================================================
  Warming up --------------------------------------
          float before   217.547k i/100ms
           float after   298.106k i/100ms
  Calculating -------------------------------------
          float before      2.157M (± 0.8%) i/s -     10.877M in   5.043292s
           float after      2.991M (± 0.6%) i/s -     15.203M in   5.082806s

  Comparison:
           float after:  2991262.8 i/s
          float before:  2156940.2 i/s - 1.39x  (± 0.00) slower

  ========================================================================
  Warming up --------------------------------------
  immutable_string before
                         163.287k i/100ms
  immutable_string after
                         298.245k i/100ms
  Calculating -------------------------------------
  immutable_string before
                            1.652M (± 0.7%) i/s -      8.328M in   5.040855s
  immutable_string after
                            3.022M (± 0.9%) i/s -     15.210M in   5.033151s

  Comparison:
  immutable_string after:  3022313.3 i/s
  immutable_string before:  1652121.7 i/s - 1.83x  (± 0.00) slower

  ========================================================================
  Warming up --------------------------------------
        integer before   115.383k i/100ms
         integer after   159.702k i/100ms
  Calculating -------------------------------------
        integer before      1.132M (± 0.8%) i/s -      5.769M in   5.095041s
         integer after      1.641M (± 0.5%) i/s -      8.305M in   5.061893s

  Comparison:
         integer after:  1640635.8 i/s
        integer before:  1132381.5 i/s - 1.45x  (± 0.00) slower

  ========================================================================
  Warming up --------------------------------------
         string before   163.061k i/100ms
          string after   299.885k i/100ms
  Calculating -------------------------------------
         string before      1.659M (± 0.7%) i/s -      8.316M in   5.012609s
          string after      2.999M (± 0.6%) i/s -     15.294M in   5.100008s

  Comparison:
          string after:  2998956.0 i/s
         string before:  1659115.6 i/s - 1.81x  (± 0.00) slower

  ========================================================================
  Warming up --------------------------------------
           time before    98.250k i/100ms
            time after   133.463k i/100ms
  Calculating -------------------------------------
           time before    987.771k (± 0.7%) i/s -      5.011M in   5.073023s
            time after      1.330M (± 0.5%) i/s -      6.673M in   5.016573s

  Comparison:
            time after:  1330253.9 i/s
           time before:   987771.0 i/s - 1.35x  (± 0.00) slower
  ```