Add custom pluralization to FastGettext

What does this MR do and why?

This speeds up pluralization when using n_(...) by 17x.

It also side-steps any potential security concerns related to eval in FastGettext.

Example failure message

Each locales must have a pluralization mapping otherwise the specs will fail with the following error:

  2) Gitlab::I18n::Pluralization.call with available locales locale: "de" supports pluralization
     Failure/Error:
               raise ArgumentError, <<~MESSAGE
                 Missing pluralization rule for locale #{locale.inspect}.

                 #{suggestion}
               MESSAGE

     ArgumentError:
       Missing pluralization rule for locale "de".

       Add the following line to /home/peter/devel/gitlab/gdk/gitlab/lib/gitlab/i18n/pluralization.rb:

         MAP = {
           ...
           "de" => ->(n) { (n != 1) },
           ...
         }.freeze

       This rule was extracted from /home/peter/devel/gitlab/gdk/gitlab/locale/de/gitlab.po.
     # ./lib/gitlab/i18n/pluralization.rb:53:in `block in <module:Pluralization>'
     # ./lib/gitlab/i18n/pluralization.rb:63:in `fetch'
     # ./lib/gitlab/i18n/pluralization.rb:63:in `call'
     # ./spec/lib/gitlab/i18n/pluralization_spec.rb:9:in `block (3 levels) in <top (required)>'
     # ./spec/lib/gitlab/i18n/pluralization_spec.rb:22:in `block (5 levels) in <top (required)>'
     # ./lib/gitlab/i18n.rb:107:in `with_locale'
     # ./spec/lib/gitlab/i18n/pluralization_spec.rb:13:in `block (4 levels) in <top (required)>'
     # ./spec/support/system_exit_detected.rb:7:in `block (2 levels) in <top (required)>'

Benchmark

Comparison:
         plural fast:   717904.2 i/s
         plural slow:    40457.3 i/s - 17.74x  slower

Code + Results

# frozen_string_literal: true

require "benchmark/ips"

Gitlab::I18n::Pluralization.install! if ENV['FAST']

p FastGettext.pluralisation_rule

FastGettext.locale = "pl_PL"

p n_("%d day", "%d days", 0)
p n_("%d day", "%d days", 1)
p n_("%d day", "%d days", 5)

Benchmark.ips do |x|
  name = "plural #{ENV['FAST'] ? 'fast' : 'slow'}"

  x.report name do
    n_("day", "days", 1)
  end

  x.compare!
  x.save! 'fast_gettext'
end

$ bin/rails runner foo.rb
WARNING: This version of GitLab depends on gitlab-shell 14.17.0, but you're running 14.15.0. Please update gitlab-shell.
#<Proc:0x0000565246e3f6e0 /home/peter/.dotfiles/asdf/installs/ruby/3.0.5/lib/ruby/gems/3.0.0/gems/fast_gettext-2.1.0/lib/fast_gettext/storage.rb:17 (lambda)>
"%d dni"
"%d dzień"
"%d dni"
Warming up --------------------------------------
         plural slow     4.125k i/100ms
Calculating -------------------------------------
         plural slow     40.457k (±15.2%) i/s -    193.875k in   5.045078s

Comparison:
         plural fast:   512635.9 i/s
         plural slow:    40457.3 i/s - 12.67x  slower

peter@happy ~/devel/gitlab/gdk/gitlab on (pl-gettext-pluralization ?M)
$ FAST=1 bin/rails runner foo.rb
WARNING: This version of GitLab depends on gitlab-shell 14.17.0, but you're running 14.15.0. Please update gitlab-shell.
Gitlab::I18n::Pluralization
"%d dni"
"%d dzień"
"%d dni"
Warming up --------------------------------------
         plural fast    65.010k i/100ms
Calculating -------------------------------------
         plural fast    717.904k (±11.2%) i/s -      3.576M in   5.040367s

Comparison:
         plural fast:   717904.2 i/s
         plural slow:    40457.3 i/s - 17.74x  slower

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.