Fully translate closures, also handle closure casts

[N1ark]

• Add TypeDecl.src: ItemKind, with the type's source (either TopLevel or Closure)
• Add fn_once_impl: TraitImplRef, fn_mut_impl: Option<TraitImplRef>, fn_impl: Option<TraitImplRef> to ClosureInfo
• If a closure type is cast to a function pointer, generate a new function that doesn't take in a state (ie. that matches the target function pointer type), to avoid having to special case closures in those casts.

This also means that closure's functions/traits are translated even when they're not used, however that's necessary to properly translate e.g. closure-casts.

For instance for the following code, if we don't translate the closure's function then it is never translated in the (U)LLBC, as it is never referred to directly (instead, only the ADT was translated).

fn main() {
    let f: fn() = || {
        let _ = 1 / 0;
    };
    f();
}

Now instead we generate all three Fn[Once,Mut] trait impls, functions, as well as an additional function that looks like

fn main::closure::as_fn(args: ()) {
  let closure_st = main::closure {};
  {impl FnOnce<()> for main::closure}::call_once(closure_st, args)
}

[N1ark]

Reworked a bit according to what you said @Nadrieril , getting some errors with generics (missing regions in ClosureInfo), should maybefn_mut_impl and fn_impl be behind a region binder?... or maybe we skip ClosureInfos when checking for generics because really it doesn't matter? idk

[Nadrieril]

Reworked a bit according to what you said @Nadrieril , getting some errors with generics (missing regions in ClosureInfo), should maybefn_mut_impl and fn_impl be behind a region binder?... or maybe we skip ClosureInfos when checking for generics because really it doesn't matter? idk

All three of them should be behind a region binder indeed, that binders the same regions as the PolyFnSig that corresponds to the closure.