nib: use new ocaml-topology interface

Depends on frenetic-lang/ocaml-topology#31
Export Nib module

Added Nib.mli. I don't know how to write the signature without the
refactorings I made in Nib.ml.
nib: node constraint
nib: eliminate nib and topology discovery

This is getting moved up to the frenetic level.
Merge pull request #104 from frenetic-lang/nib

Eliminate topology discovery code