remove the code_builder

this is core of #11
rename compiler_builder to just builder
and change all builder uses to use that
some test change as code is not returned anymore

test/risc/test_builder2.rb

@@ -8,7 +8,7 @@ module Risc
       Risc.boot!
       @init = Parfait.object_space.get_init
       @compiler = Risc::MethodCompiler.new( @init )
-      @builder  = @compiler.compiler_builder(@init)
+      @builder  = @compiler.builder(@init)
     end
     def test_list
       assert_equal :List , @builder.infer_type(:list).class_name