Prim.entersRuntime vs Prim.runtimeTransfer

[Matthew Fluet]

> I'm not quite sure what to do about MLton_halt -- probably the right
> thing is to add it as another kind of transfer, with no return label.

One of the reasons that I made MLton_halt a runtime transfer was the
thought that we could adjust removeUnused to make everything following the
MLton_halt unused/unreachable.  It would use a 'bug block' for the return
label (like it does with non-tail calls to functions that can't return,
but could raise to a handler).  Again, no big win at all, other than the
satisfaction that the empty program wouldn't have three invocations of
MLton_halt.