Docs: update exceptions proposal fuzzing and C-API status.

[cfallin]

Noticed while reviewing another docs update: we still state that the exceptions proposal is not fuzzed, and has no C API support. Neither is true anymore, AFAIK:

• We fuzz exceptions via our usual infrastructure and we had a number of fuzzbugs come in when we first merged support.
• We added C API support for enabling exceptions in Expose exceptions proposal configuration on C API #11861.

[cfallin]

(This'll be a merge conflict with #12036, sorry Alex! And if you disagree that we have enough fuzzing to check the box here I'm happy to remove that part; we don't have a specific target of course but we do fuzz it overall)

[alexcrichton]

For the C API I believe one missing piece is bindings for exnref values. I don't think that the type is plumbed in the API yet nor the ability to create/manage the references. I think all the pieces are there, however, and it's mostly just a matter of copying the anyref constructors/inspectors/etc for exnref as well.

For fuzzing I could go sort of either way myself. On one hand the preexisting wasm-smith-based fuzzing is good in that it creates interesting shapes of code. In that sense I feel like it fuzzes the compiler side of exceptions relatively well. On the other hand though, like with most other wasm-smith-based-fuzzing, I feel like it's probably leaving quite a lot on the table in terms of runtime-handling fuzzing. For example I suspect everything either immediately throws-and-doesn't-catch or never throws because intersting payloads are never on the stack (or something like that). In that sense I do think there's very real space for a more interesting fuzzer exceptions-wise, primarily focused on the runtime behavior of exceptions as opposed to the compile-time-behavior. I don't have a concrete idea though what such a fuzzer would precisely be fuzzing, and whether we block on that for enabling exceptions, I'm not sure. Perhaps a good meeting topic as well?

At the very least though adding a 🚧 symbol plus a footnote explaining the current situation is definitely ok though.

[cfallin]

Good points! I had completely forgotten about the host-API-creates-exnrefs side of the C API -- indeed we'll want that. Updated both host API and fuzzing to 🚧 and we can talk about fuzzing more in the next meeting.