Enforce where-clauses on type aliases via alias type

[nikomatsakis]

This is the types team tracking issue for the idea of changing type aliases from a "early expanded hack" in the compiler into a simple form of alias type (one that can always be normalized). This would allow us to enforce aliases and potentially improve diagnostics.

Blocked on lang-team, who needs to decide whether we want to do this change and especially the desired semantics for implied bounds.

cc rust-lang/rust#112792

[nikomatsakis]

This was discussed at the types team 2023 meetup; notes can be found here. The primary outcome was that we identified that @rust-lang/lang decision was needed and filed rust-lang/lang-team#227.

[fmease]

No longer part of Rust '24 but still actively being worked on by me.