[Goal]: unify representations of `ConstKind::Unevaluated` and `TyKind::Alias` and allow non defid/subst pairs

[BoxyUwU]

also includes merging ConstKind::Expr into ConstKind::Unevaluated

[khyperia]

I am starting to work on this ✨ - anyone other than boxy, feel free to ping me on zulip if I'm quiet/seemed to have stopped working on this and you want to take over (boxy must pong me instead of pinging me)

let's see if rustbot works in this repo...

@rustbot claim