[WIP] Implement subroutine opcodes

eth/vm/logic/flow.py

@@ -7,6 +7,7 @@
 from eth.vm.computation import BaseComputation
 from eth.vm.opcode_values import (
     JUMPDEST,
+    BEGINSUB,
 )
 
 
@@ -57,3 +58,40 @@ def gas(computation: BaseComputation) -> None:
     gas_remaining = computation.get_gas_remaining()
 
     computation.stack_push_int(gas_remaining)
+
+
+def beginsub(computation: BaseComputation) -> None:
+    #TODO: raise OOG exception
+    pass
+
+
+def jumpsub(computation: BaseComputation) -> None:
+    sub_loc = computation.stack_pop1_int()
+
+    temp = computation.code.program_counter
+    computation.code.program_counter = sub_loc
+
+    next_opcode = computation.code.peek()
+
+    if next_opcode != BEGINSUB:
+        #TODO: abort execution
+        pass
+
+    else:
+        computation.code.program_counter += 1
+
+    if computation.rstack.length >= 1023:
+        #TODO: abort execution
+        pass
+
+    computation.rstack_push_int(temp + 1)
+
+
+def returnsub(computation: BaseComputation) -> None:
+
+    if computation.rstack.length == 0:
+        #TODO: abort execution
+        pass
+
+    computation.code.program_counter = computation.rstack_pop1_int()
+