Extract all instance shim variants into new `ShimKind` enum They tend to have similar handling -- e.g., they should be the only input to the `mir_shims` query -- so it's cleaner to group them together. This will also make potential future refactorings easier, such as only carrying `GenericArgsRef` for instances that actually use it (e.g., `Item`) but not others (e.g., `CloneShim`). cc https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/moving.20ty.3A.3AInstance.2Eargs.20into.20ty.3A.3AInstanceKind.20variants r? @oli-obk