contification

[Matthew Fluet]

> I think that the right solution is to re-introduce HandlerPush and
> HandlerPop, for the purposes of the paper, so that we can use the
> handler info from the source program, and easily propagate it.  All
> SSA optimizations just need to preserve Push and Pop, and will never
> see Set and friends.

I would still like to keep explicit handler fields in non-tail calls.
That's essential now for the way contification works (not to mention all
the stub's in backend).  This shouldn't be a problem, we can push and pop
without making any non-tail calls that use the handler.