Complete support for Wasm GC

I'm currently using `waffle` to build a toolchain which will target Wasm GC; however, I noticed that support for this proposal is not fully complete. Is this a feature for which you'd be willing to accept pull requests?

Aside from GC, waffle is working great for my use-case, so thanks! As I started adopting it though, I realized that some of the [frontend bits are only visible to the crate](https://github.com/bytecodealliance/waffle/blob/main/src/lib.rs#L26); is that an intentional design decision? For my use-case I ended up building a generic builder on top of the IR bits inspired on the frontend bits (and Simple And Efficient Construction of SSA Form). I was wondering if providing public APIs à la Cranelift would be something that you'd be interested in?

I'm mixing apples and oranges here, so happy to open a different issue for the builder API if preferred.