Introducing the "lookup-constants runtime system" :)

[Stephen Weeks]

> 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.

I'm waffling.  I'm now leaning towards leaving most _prim's as is, but for the 3
or 4 special constants settable on the command line, introducing a new syntax,
_host_const.  OK?