Rename singleton subclass instance helper

lib/mutant/mutation/filter.rb

@@ -54,7 +54,7 @@ module Mutant
       end
 
       # Mutation filter matching all mutations
-      Mutant.define_singleton_subclass('ALL', self) do
+      Mutant.singleton_subclass_instance('ALL', self) do
 
         # Test for match
         #

lib/mutant/singleton_methods.rb

@@ -10,7 +10,7 @@ module Mutant
   #
   # @api private
   #
-  def self.define_singleton_subclass(name, superclass, &block)
+  def self.singleton_subclass_instance(name, superclass, &block)
     klass = Class.new(superclass) do
 
       def inspect; self.class.name; end