Somehow support exceptions

[gkgoat1]

Rust added wasm unwinding using exceptions, so support would be great (both in the main repo and my fork)

Intruoduction: rust-lang/rust#121438

[cfallin]

@gkgoat1 if you're interested in working on this, I'd be happy to review the work! The main design question is how this manifests in the IR; we're addressing the parallel question over in Cranelift right now so I suspect we can wait to see how that looks (likely something like LLVM's invoke instruction, with two branch targets, putting the exceptional control flow into the CFG) and then follow whatever works.