fix(frontend): fix select_trait_candidate

[W95Psp]

Fixes #461 and #476

[franziskuskiefer]

This makes hax crash. For example

Fatal error: exception Failure("impl_expr_atom: Todo ImplExprAtom::Todo(see https://github.com/hacspec/hax/issues/381) ImplSourceClosureData(closure_def_id=DefId(0:673 ~ bertie[019b]::tls13utils::{impl#3}::declassify::{closure#0}), substs=[i16, extern \"rust-call\" fn((&u8,)) -> u8, ()], nested=[Obligation(predicate=Binder(ClosureKind(DefId(0:673 ~ bertie[019b]::tls13utils::{impl#3}::declassify::{closure#0}), [i16, extern \"rust-call\" fn((&u8,)) -> u8, ()], FnMut), []), depth=1)])\n\nBinder(\n    <[closure@src/tls13utils.rs:202:27: 202:30] as std::ops::FnMut<(&u8,)>>,\n    [],\n)")
Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
Called from Hax_engine__Import_thir.Make.c_impl_expr in file "lib/import_thir.ml", line 906, characters 15-44

[W95Psp]

Yes, I noticed it's producing new bugs 😅
But that is fine: those are just new cases I need to take care of, I see what is the problem, that's just some small legitimate changes to do :)

[franziskuskiefer]

you force pushed again 😔

[franziskuskiefer]

Lets get this in

[W95Psp]

Great, thank you 😃