generalize get_main and get_init to get_method

to get at those know methods that really
__must__ exists, hence the bang, raise if don't
about to add method missing and raise to the list

test/risc/test_builder2.rb

@@ -6,9 +6,9 @@ module Risc
     def setup
       Parfait.boot!(Parfait.default_test_options)
       Risc.boot!
-      @init = Parfait.object_space.get_init
-      @compiler = Risc::MethodCompiler.new( @init, Mom::Label.new( "source_name", "return_label") )
-      @builder  = @compiler.builder(@init)
+      method = FakeCallable.new
+      @compiler = Risc::MethodCompiler.new( method, Mom::Label.new( "source_name", "return_label") )
+      @builder  = @compiler.builder(method)
     end
     def test_list
       assert_equal :List , @builder.infer_type(:list).class_name