rustc: middle: ty: replace unsafe pointers with references in Ty.
rustc: fix fallout of adding the `'tcx` lifetime to `Ty`.
rustc: fix fallout of making `Ty` an alias for `&TyS` instead of a wrapper.
rustc: middle: avoid clones in ty_fn_{sig,args}.
rustc: middle: remove obsolete ty::get.
rustc: middle: simplify primitive type declaration in ty.
rustdoc: fix fallout of ty::t -> Ty<'tcx>.