Introducing the "lookup-constants runtime system" :)

[Stephen Weeks]

> Here  is what I meant by `fixed names'.  There is a small collection of names
> (like MLton_safe) which we want to get from the machine doing the compilation
> and  all  the  other  names  should come from the machine which is the target
> machine.  Instead of hard-wiring in the name MLton_safe  in  to  the  system,
> hard-wire  in  a  prefix like MLton_Flags_ and then use that as the prefix of
> all the names which we want to come from the system doing the compilation.

I prefer to express this via syntax (_host_const, _target_const) as opposed to
naming convention to make it clearer that things are being treated differently
and to catch typos.

Anyways, I'm still happier with leaving _prim as is and adding the two command
line switches (-stop constprog, -load-const) for cross-compilation.

Let me know if you guys agree and I'll send mail to Anoq.