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/mom/test_callable_compiler.rb

@@ -1,6 +1,4 @@
 require_relative "helper"
-class FakeCallable
-end
 module Mom
   class FakeCallableCompiler < CallableCompiler
     def source_name