Remove `ConstrainVarSubReg` from inference and replace with a verify

[nikomatsakis]

Region inference needs some big-scale love, but even without that it has some deprecated bits. For example, the role of ConstrainVarSubReg has really been relegated to a verification step. We ought to remove it and just use VerifyRegSubReg. I started doing this, but it gets a bit complicated in dealing with the error reporting code (which itself needs to be reworked). So I decided to put it off for now and label it a FIXME.

[nikomatsakis]

I'd be happy to mentor this work, though it's kind of in the guts of things.

[brson]

Triage: ConstrainVarSubReg still exists. Help still wanted.

[jdhorwitz]

@nikomatsakis If you are still wiling to mentor I would love to help with this. Thanks!

[nikomatsakis]

Hmm @jdhorwitz at this point I'm not sure if this refactoring is worth it. We expect to replace a lot of this code anyway. I think I'm inclined to close this issue, but I have a list of mentoring tasks that I try to keep up to date. I have been planning (for some time, I confess) to go through and for each one write up directions for each one, but if anything catches your eye, let me know.