Asyncify: spill reference-typed locals to tables

[NellowTCS]

Fixes #3739.

Right now wasm-opt --asyncify aborts if a reference-typed value is live across an unwind. Asyncify spills locals into a linear-memory stack, and references can't be stored there, so it gives up.
This was annoying for my IPC/RPC runtime, Saikuro, so I decided to go fix Asyncify.

As discussed by @kripken, @martianboy, and @surma in #3739, the fix is to spill references into tables instead of linear memory.

The Asyncify ABI/API remains unchanged, so this shouldn't impact users beyond fixing reference-type support, I believe.
Asyncify still only supports one active pause at a time, though. As a result, asyncify_start_unwind now traps if it sees a nonzero cursor.

However, multi-pause support would need per-pause regions in the tables. As pointed out, that's a separate problem and a much bigger change, so I left it out of this PR.

Note: This is essentially a cleaned-up and "productionized" version of the proof-of-concept @alexdoesh posted in #3739.

[kripken]

Back in the day we did think this would make sense eventually, but meanwhile JSPI provides what Asyncify does but in the VM itself, which is a lot more efficient. That support of course includes reference-typed locals.

It sounds like you might be using Asyncify in some other way? Can you explain the motivation and how important this is for you?

[NellowTCS]

Hi!!!
I definitely agree that JSPI does this (and does it much better), but the problem with it is adoption. Currently, it's a new Baseline feature, so a lot of devices/people don't have support for it yet.

I was planning to use this for Saikuro's executor abstraction (saikuro-exec) on its WASM path. I had implemented a block_on that prefers JSPI when available, but will eventually (using the changes in this PR) also fall back to Asyncify in environments where JSPI is unavailable.

The main issue I ran into is that reference-typed values can remain live across suspension points (which is often the case in Saikuro). In those cases Asyncify currently aborts, which makes it impossible to transform otherwise-valid modules.