Introducing the "lookup-constants runtime system" :)

[Stephen Weeks]

> Yes, the funny constants set on the MLton command line would have to be
> handled separately.  Are there very many of those?

I doubt it.  But again, they could be introduced in the user program.  So, it
looks to me like like we need _prim_host and _prim_target, or maybe just 2 new
names, since _prim is already overloaded for compiler primitives.  How about

_host_const
_target_const