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/support/fake_compiler.rb

@@ -1,3 +1,5 @@
+class FakeCallable
+end
 module Risc
   class FakeCompiler
     attr_reader :instructions