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`).

diff --cc src/librustc/metadata/encoder.rs
Simple merge

diff --cc src/librustc/metadata/tydecode.rs
Simple merge

diff --cc src/librustc/metadata/tyencode.rs
Simple merge

diff --cc src/librustc/middle/astencode.rs
Simple merge

diff --cc src/librustc/middle/lint.rs
Simple merge

diff --cc src/librustc/middle/trans/base.rs
Simple merge

diff --cc src/librustc/middle/trans/consts.rs
Simple merge

diff --cc src/librustc/middle/trans/debuginfo.rs
Simple merge

diff --cc src/librustc/middle/trans/reflect.rs
Simple merge

diff --cc src/librustc/middle/ty.rs
Simple merge

diff --cc src/librustc/middle/ty_fold.rs
Simple merge

diff --cc src/librustc/middle/typeck/astconv.rs
Simple merge

diff --cc src/librustc/middle/typeck/check/method.rs
Simple merge

diff --cc src/librustc/middle/typeck/check/mod.rs
Simple merge

diff --cc src/librustc/middle/typeck/infer/coercion.rs
Simple merge

diff --cc src/librustc/middle/typeck/infer/combine.rs
Simple merge

diff --cc src/librustc/util/ppaux.rs
Simple merge