projections in impl headers

[lcnr]

Summary

Talk about rust-lang/rust#107263 and rust-lang/rust#99554 and whether we want to change the orphan check to run after normalization.

This impacts both inherent and trait impls

Rationale

Have to make progress on rust-lang/rust#99554 and fixing this without normalizing as part of the orphan check results in too much breakage.

[lcnr]

discussed during the in person meetup, we will go ahead and normalize before the orphan check. The PR doing so will require an FCP.