[MLton] MLB path variables (was: mlnlffigen is awesome)

[Stephen Weeks]

> What I had in mind is that path variables would work just like other
> bindings; Path variable bindings imported from a basis would only
> effect paths in the scope of the import.

This makes sense.  It still seems a little tricky to get at SML_LIB
and other things that are not part of the current project, and may be
in different places on different machines.  Would one have a Makefile
that generates the appropriate MLB that defines SML_LIB, and then
import that into every MLB that refers to SML_LIB?  Or would there
still be some special variables that are in the primitive basis that
MLBs are elaborated in?  At which point I'm wondering if this proposal
is complementary to the current MLB path variables rather than a
replacement.  Namely, your proposal gives one a way to name path
variables for use within a project, while the current MLB path
variables are for referring to other projects by a portable name.