Ruby 3.3 allows passing a list of characters to
`SecureRandom.alphanumeric`. For `SecureRandom.base36` using `choose` is
faster than the current implementation. For `SecureRandom.base58` it is
a bit slower.

I've also added a test to make sure passing nil as the length defaults
the length to 16.

_Benchmark__

```ruby

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", github: "rails/rails", branch: "main"
  gem "benchmark-ips"
end

require "active_support"
require "active_support/core_ext/securerandom"

module SecureRandom
  def self.fast_base36(n)
    alphanumeric(n, chars: BASE36_ALPHABET)
  end
end

[10, 100, 1000, 10000].each do |length|
  puts
  puts " #{length} ".center(80, "=")
  puts

  Benchmark.ips do |x|
    x.report("base36")      { SecureRandom.base36(length) }
    x.report("fast_base36") { SecureRandom.fast_base36(length) }
    x.compare!
  end
end
```

```
====================================== 10 ======================================

Warming up --------------------------------------
              base36    20.513k i/100ms
         fast_base36    24.843k i/100ms
Calculating -------------------------------------
              base36    200.940k (±13.8%) i/s -    984.624k in   5.060203s
         fast_base36    235.531k (± 5.7%) i/s -      1.192M in   5.080574s

Comparison:
         fast_base36:   235530.9 i/s
              base36:   200939.9 i/s - same-ish: difference falls within error

===================================== 100 ======================================

Warming up --------------------------------------
              base36     2.746k i/100ms
         fast_base36     2.995k i/100ms
Calculating -------------------------------------
              base36     25.559k (± 8.5%) i/s -    129.062k in   5.087961s
         fast_base36     30.265k (± 6.6%) i/s -    152.745k in   5.070263s

Comparison:
         fast_base36:    30264.7 i/s
              base36:    25558.8 i/s - 1.18x  slower

===================================== 1000 =====================================

Warming up --------------------------------------
              base36   278.000  i/100ms
         fast_base36   308.000  i/100ms
Calculating -------------------------------------
              base36      2.595k (±11.6%) i/s -     12.788k in   5.007921s
         fast_base36      3.133k (± 6.1%) i/s -     15.708k in   5.033310s

Comparison:
         fast_base36:     3132.6 i/s
              base36:     2594.9 i/s - 1.21x  slower

==================================== 10000 =====================================

Warming up --------------------------------------
              base36    24.000  i/100ms
         fast_base36    34.000  i/100ms
Calculating -------------------------------------
              base36    256.601  (± 8.6%) i/s -      1.296k in   5.089604s
         fast_base36    322.119  (± 6.5%) i/s -      1.632k in   5.089614s

Comparison:
         fast_base36:      322.1 i/s
              base36:      256.6 i/s - 1.26x  slower

```