Merge #8018

8018: Make Ty wrap TyKind in an Arc r=flodiebold a=flodiebold

... to further move towards Chalk.

This is a bit of a slowdown (218ginstr vs 213ginstr for inference on RA), even though it allows us to unwrap the Substs in `TyKind::Ref` etc..

Co-authored-by: Florian Diebold <flodiebold@gmail.com>

diff --cc crates/hir/src/lib.rs
Simple merge

diff --cc crates/hir_ty/src/infer.rs
Simple merge

diff --cc crates/hir_ty/src/infer/expr.rs
Simple merge