Add rez-bisect tool

It would be really quite handy in situations where a package resolve causes a problem, to be able to run a command similar to git-bisect [i.e.] rez-bisect where a binary search of package-resolutions could be run with a script or command to be run to check validity, as a way of locating a difficult-to-trace issue in a complex tree of dependencies.

The issue names no files, tests, or entry points. Start by locating Rez's command-line and package-resolution entry points, then define how a validation script or command drives the search. Done means a usable rez-bisect command can binary-search package resolutions and identify the problematic dependency set.

Written by the indexing model from the issue text.