Tue May 3 10:20:09 EDT 2011

Simplified op_prim

Actually, OP_PRIM does not need to call return.  It can just continue
reducing the next code atom, since it has no need of touching the
environment during eval.

The op_prim that's there now is not a primitive in the ordinary sense,
but a VM extension.  To make the above remark usable, a real prim op
needs to perform argument reference too, but can then skip closure