Optimize access to the en inflections

The `en` inflector is almost always the one used for itnernal
framework inflections, hence it's called much more than any other.

Hence it's worth having a fast path for it.

Before:

```
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Calculating -------------------------------------
             regular    278.450k (± 1.1%) i/s    (3.59 μs/i) -      1.401M in   5.030803s
           irregular      1.320M (± 0.9%) i/s  (757.36 ns/i) -      6.617M in   5.012038s
         uncountable      2.417M (± 0.5%) i/s  (413.72 ns/i) -     12.172M in   5.035810s
```

After:

```
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +YJIT +PRISM [arm64-darwin24]
Calculating -------------------------------------
             regular    297.666k (± 1.3%) i/s    (3.36 μs/i) -      1.509M in   5.068638s
           irregular      1.975M (± 0.6%) i/s  (506.28 ns/i) -      9.939M in   5.031926s
         uncountable      6.078M (± 0.5%) i/s  (164.54 ns/i) -     30.504M in   5.019120s
```

Benchmark:

```
require "bundler/inline"

gemfile do
  gem "rails", path: "."
  gem "benchmark-ips"
end

Benchmark.ips do |x|
  x.report("regular") { ActiveSupport::Inflector.pluralize("test") }
  x.report("irregular") { ActiveSupport::Inflector.pluralize("zombie") }
  x.report("uncountable") { ActiveSupport::Inflector.pluralize("sheep") }
end
```

Author: byroot

activesupport/lib/active_support/inflector/inflections.rb

@@ -30,6 +30,7 @@ module Inflector
     # before any of the rules that may already have been loaded.
     class Inflections
       @__instance__ = Concurrent::Map.new
+      @__en_instance__ = nil
 
       class Uncountables # :nodoc:
         include Enumerable
@@ -74,10 +75,14 @@ def uncountable?(str)
       end
 
       def self.instance(locale = :en)
+        return @__en_instance__ ||= new if locale == :en
+
         @__instance__[locale] ||= new
       end
 
       def self.instance_or_fallback(locale)
+        return @__en_instance__ ||= new if locale == :en
+
         I18n.fallbacks[locale].each do |k|
           return @__instance__[k] if @__instance__.key?(k)
         end

activesupport/test/inflector_test.rb

@@ -16,12 +16,16 @@ def setup
     # This helper is implemented by setting @__instance__ because in some tests
     # there are module functions that access ActiveSupport::Inflector.inflections,
     # so we need to replace the singleton itself.
-    @original_inflections = ActiveSupport::Inflector::Inflections.instance_variable_get(:@__instance__)[:en]
-    ActiveSupport::Inflector::Inflections.instance_variable_set(:@__instance__, en: @original_inflections.dup)
+    @original_inflections = ActiveSupport::Inflector::Inflections.instance_variable_get(:@__instance__)
+    @original_inflection_en = ActiveSupport::Inflector::Inflections.instance_variable_get(:@__en_instance__)
+
+    ActiveSupport::Inflector::Inflections.instance_variable_set(:@__instance__, {})
+    ActiveSupport::Inflector::Inflections.instance_variable_set(:@__en_instance__, @original_inflection_en.dup)
   end
 
   def teardown
-    ActiveSupport::Inflector::Inflections.instance_variable_set(:@__instance__, en: @original_inflections)
+    ActiveSupport::Inflector::Inflections.instance_variable_set(:@__instance__, @original_inflections)
+    ActiveSupport::Inflector::Inflections.instance_variable_set(:@__en_instance__, @original_inflection_en)
   end
 
   def test_pluralize_plurals