[WIP] Implement subroutine opcodes

Author: Giuseppe Cerami

eth/abc.py

@@ -1594,13 +1594,13 @@ def stack_dup(self, position: int) -> None:
     # Return Stack Managemement
     #
     @abstractmethod
-    def rstack_push_int(self) -> Callable[[int], None]:
+    def rstack_push_int(self) -> None:
         """
         Push integer onto the return stack.
         """
 
     @abstractmethod
-    def rstack_pop1_int(self) -> Callable[[int], None]:
+    def rstack_pop1_int(self) -> int:
         """
         Pop integer off the return stack and return it.
         """