Sun May 1 15:05:24 EDT 2011

One continuation type

It seems te best optimization to make is to remove dependency on
multiple continuation types, which means that `begin' is implemented
in terms of the `let' primitive, but followed by a pop which
explicitly ignores the retval.

DROP + LET : which order? Drop after let + make sure the env is passed

So, now that begin is eliminated, the continuation can be simplified.
I don't really need a K_HALT probably.  Next: eliminate K_HALT.  It
can be implemented as a return frame with a halt opcode.

Done.  That wasn't so hard!