And benchmark with this branch for immutable string type:

```ruby
ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name
    t.string :fast_name
  end
end

class User < ActiveRecord::Base
  attribute :fast_name, :immutable_string
end

user = User.new

Benchmark.ips do |x|
  x.report("user.name") do
    user.name = "foo"
    user.name_changed?
  end
  x.report("user.fast_name") do
    user.fast_name = "foo"
    user.fast_name_changed?
  end
end
```

```
Warming up --------------------------------------
           user.name    34.811k i/100ms
      user.fast_name    39.505k i/100ms
Calculating -------------------------------------
           user.name    343.864k (± 3.6%) i/s -      1.741M in   5.068576s
      user.fast_name    384.033k (± 2.7%) i/s -      1.936M in   5.044425s
```