Hinting

[benthestatistician]

@adamrauh's hinting branch is roughly ready to be pulled back into the main optmatch repo, so I'm starting this pull request. Stuff we'd like to have done -- add to this list --

• [ ] Hinting is not enabled for fallback solver. I don't think it should be, but ideally we should throw an informative error if/when user is using fallback solver and tries to pass a hint.
• [ ] Review/update names of arguments used to pass hinting info . (At this moment, "node.prices"; "start" seems better, and perhaps there's better yet.)

[benthestatistician]

Since this request was posted, Adam made a fresh start on a fresh branch. Accordingly I'll close this merge request while we expect another merge request from that different branch.