Optimize normalize_flat_keys

I18n::Backend::Flatten.normalize_flat_keys :en, :home, [:country, :county, :second], '|'

Calculating -------------------------------------
            original     1.060k memsize (     0.000  retained)
                        21.000  objects (     0.000  retained)
                         7.000  strings (     0.000  retained)
               patch   652.000  memsize (     0.000  retained)
                        14.000  objects (     0.000  retained)
                         7.000  strings (     0.000  retained)

Comparison:
               patch:        652 allocated
            original:       1060 allocated - 1.63x more

Warming up --------------------------------------
            original    15.765k i/100ms
               patch    18.043k i/100ms
Calculating -------------------------------------
            original    156.733k (± 3.5%) i/s -    788.250k in   5.036161s
               patch    180.959k (± 1.9%) i/s -    920.193k in   5.086948s

Comparison:
               patch:   180959.0 i/s
            original:   156733.1 i/s - 1.15x  (± 0.00) slower

lib/i18n/backend/flatten.rb

@@ -18,14 +18,17 @@ module Flatten
       # and creates way less objects than the one at I18n.normalize_keys.
       # It also handles escaping the translation keys.
       def self.normalize_flat_keys(locale, key, scope, separator)
-        keys = [scope, key].flatten.compact
+        keys = [scope, key]
+        keys.flatten!
+        keys.compact!
+
         separator ||= I18n.default_separator
 
         if separator != FLATTEN_SEPARATOR
-          keys.map! do |k|
-            k.to_s.tr("#{FLATTEN_SEPARATOR}#{separator}",
-              "#{SEPARATOR_ESCAPE_CHAR}#{FLATTEN_SEPARATOR}")
-          end
+          from_str = "#{FLATTEN_SEPARATOR}#{separator}"
+          to_str = "#{SEPARATOR_ESCAPE_CHAR}#{FLATTEN_SEPARATOR}"
+
+          keys.map! { |k| k.to_s.tr from_str, to_str }
         end
 
         keys.join(".")