Use constant nodes when possible

lib/mutant/mutator/node.rb

@@ -174,7 +174,7 @@ module Mutant
       # @api private
       #
       def emit_nil
-        emit(s(:nil))
+        emit(N_NIL)
       end
 
       # Return new self typed child

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

@@ -16,7 +16,7 @@ module Mutant
           # @api private
           #
           def dispatch
-            emit(s(:send, s(:const, s(:cbase), :Object), :new))
+            emit(NEW_OBJECT)
           end
 
         end # Nil

lib/mutant/node_helpers.rb

@@ -19,6 +19,10 @@ module Mutant
     NAN               = s(:send, s(:float,  0.0), :/, s(:args, s(:float, 0.0)))
     NEGATIVE_INFINITY = s(:send, s(:float, -1.0), :/, s(:args, s(:float, 0.0)))
     INFINITY          = s(:send, s(:float,  1.0), :/, s(:args, s(:float, 0.0)))
+    NEW_OBJECT        = s(:send, s(:const, s(:cbase), :Object), :new)
+
+    N_NIL             = s(:nil)
+    N_EMPTY           = s(:empty)
 
   end # NodeHelpers
 end # Mutant