Upgrade toolchain to nightly-2026-04-21

[Lysxia]

• instantiate wraps its result in Unnormalized<_> so this is a good opportunity to revisit how we handle normalization
• DefKind::Const and AssocConst now carry a boolean to keep track of whether they are "suitable for generics", if I understand correctly. I don't think it affects us (hence this PR just ignores that boolean), but while looking into it I learned that people working on const generics have at least thought about having a termination checker to enable more powerful const generics: https://hackmd.io/OsVAAEgOS2ajDwxg44kN-A?view
• the only coma diff shows that AtomicBool has become a synonym for Atomic<Bool> (etc.) which seems relevant to concurrency folks core: make atomic primitives type aliases of Atomic<T> rust-lang/rust#153015

[Lysxia]

I've been looking into removing Namer::normalize (as there is a TODO saying to remove it), the following uses remain:

• normalize RPITs in the result of a function
• normalize ADT field types

The first one seems not too hard to avoid, but the second seems more essential.

I got rid of other uses by pushing normalization to the translation phase (MIR -> FMIR and Pearlite THIR -> Term), although it feels rather fragile... The tests pass but I wouldn't be surprised if there are still terms containing unnormalized or unerased types that we just happen to not look at.

[Lysxia]

I think I'm about done cleaning up the normalizes for this PR.