Move more literals to constants

As constans the literals get frozen. This improves sharpness of API use.

lib/mutant.rb

@@ -20,7 +20,9 @@ require 'concord'
 
 # Library namespace
 module Mutant
-end
+  # The empty string used within this namespace
+  EMPTY_STRING = ''.freeze
+end # Mutant
 
 require 'mutant/cache'
 require 'mutant/node_helpers'

lib/mutant/cli/classifier.rb

@@ -10,7 +10,8 @@ module Mutant
       OPERATOR_PATTERN    = Regexp.union(*OPERATOR_METHODS.map(&:to_s)).freeze
       METHOD_NAME_PATTERN = /([_A-Za-z][A-Za-z0-9_]*[!?=]?|#{OPERATOR_PATTERN})/.freeze
       SCOPE_PATTERN       = /(?:::)?#{SCOPE_NAME_PATTERN}(?:::#{SCOPE_NAME_PATTERN})*/.freeze
-
+      CBASE_PATTERN       = /\A::/.freeze
+      SCOPE_OPERATOR      = '::'.freeze
       SINGLETON_PATTERN   = %r(\A(#{SCOPE_PATTERN})\z).freeze
 
       REGISTRY = []
@@ -35,7 +36,7 @@ module Mutant
       # @api private
       #
       def self.constant_lookup(location)
-        location.gsub(%r(\A::), '').split('::').inject(Object) do |parent, name|
+        location.gsub(CBASE_PATTERN, EMPTY_STRING).split(SCOPE_OPERATOR).inject(Object) do |parent, name|
           parent.const_get(name)
         end
       end

lib/mutant/mutator/node/literal/regex.rb

@@ -7,8 +7,6 @@ module Mutant
 
           handle(:regexp)
 
-          EMPTY_STRING = ''.freeze
-
           # No input can ever be matched with this
           NULL_REGEXP_SOURCE = 'a\A'.freeze