Move mutability from ty_vec and ty_trait to their respective stores.

[eddyb]

Cleans up some remnants of the old mutability system and only allows vector/trait mutability in VstoreSlice (&mut [T]) and RegionTraitStore (&mut Trait).

[flaper87]

@nikomatsakis you may be interested in looking at this PR.

[huonw]

I'm not totally sold on the generic Vstore<M = Mutability> thing. It's very nice for expressing that strings are immutable... but there's the (small) risk of things becoming generic and bloating code like crazy (see the bloat caused by making the pretty printer generic for an extreme example).

[huonw]

(And there's a few places where there's some mildly annoying duplication due to it. :( )

[nikomatsakis]

This looks pretty nice! btw, thanks for factoring things into their own commits, that was helpful.

[nikomatsakis]

r+ from me once nits are fixed

[eddyb]

@huonw The pretty printer was causing bloat due to 4 (if I'm not mistaken) implementations in rustc.
There are only two possible instantiations of Vstore - anyways, all of this will be swept under the rug by DST :).

[brson]

Failure looks legit.