make `RegionVid` use `newtype_index!`

[nikomatsakis]

So @spastorino and @Nashenas88 have been on this awesome kick of making all "index-like things" use newtype_index!. In #45825, I made RegionVid implement Idx, but I didn't use newtype_index! because -- well, because dang it that PR is long enough. It'd be nice to do.

(I think btw we should shoot for not using public fields when we do these conversions -- I'd like to be able to transparently start using Nonzero to make Option<I>, as described in #45842.)

[nikomatsakis]

Done in nikomatsakis#11.