rename typed_method to callable_method

seems to make the essence clearer
also extracted base class

lib/risc/instruction.rb

@@ -27,7 +27,7 @@ module Risc
       @next = nekst
       return unless source
       raise "Source must be string or Instruction, not #{source.class}" unless source.is_a?(String) or
-                source.is_a?(Mom::Instruction) or source.is_a?(Parfait::TypedMethod)
+                source.is_a?(Mom::Instruction) or source.is_a?(Parfait::CallableMethod)
     end
     attr_reader :source