enter only via jumpsub

[gcolvin]

As proposed by Pawel, Andrei and Alex @chfast @gumb0 @axic

Change the specification in a way that BEGINSUB can only be reached via JUMPSUB . >
Specifically:

1. Execution of BEGINSUB causes exception (OOG: all gas consumed) and terminates execution. This way BEGINSUB behaves like INVALID (aka 0xfe ) and it should never be executed in a well-formed EVM program.
2. JUMPSUB sets the pc to location + 1 ( As opposed to location in the current spec ).
3. In the edge case when BEGINSUB is the last instruction in code and this subroutine is jumped-to, the implementations should execute STOP . This is consistent with the other similar case of returning from a subroutine jumped-to from the JUMPSUB being the last instruction in code.