[META] Frontend completeness

[W95Psp]

We want the frontend of hax to be complete, that is:

• all Rust constructs should be handled;
• the frontend shall not crash.

Currently the main pain with frontend is traits: we implement our own logic for finding implementations for traits. This part is the only "smart" part of the frontend, I would say.
This trait implementation resolver is the only part of the frontend where we (1) go deep down rustc's AST (2) reconstruct some complicated data and ask Rust something. Here "reconstruct some complicated data" involves binders: we have a hard time reconstructing binders correctly, thus we break invariants and rustc crashes.

List of bugs:

• Panic: type parameter out of range when instantiating #747
• aes crate: the frontend loops indefinitely #686
• Frontend crash: ImplExprPredNotFound on bounds on opaque types #684
• Unreachable place in Rust AST on tvix/nix-compat - ImplExprPredNotFound #604
• Frontend: panic on default traits parameters #310

Ccing @R1kM @Nadrieril @sonmarcho who are probably interested

Action items

• Catch those failures and print nice warnings
• Add item-wise catches? to improve UX
• Frontend: tracing-tree: improve usability #944
• How long would that take?