rustc: rename ty::vstore and its variants to UpperCamelCase.
rustc: use VstoreFixed's length in crate-independent type hashes.
rustc: move mutability from ty_vec and ty_trait to VstoreSlice and RegionTraitStore.
rustc: fix the fallout from moving mutability into VstoreSlice and RegionTraitStore.
auto merge of #13424 : eddyb/rust/ty-mut-in-store, r=nikomatsakis

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