Improve loading speed

[technosophos]

Right now, WAGI loads the WASM module at request time. Big WASM modules can take seconds to load.

There is probably a way to cache or preload modules to speed this up. wasm3 interprets rather than JIT-ing, and that might be substantially faster in this case (though I don't think wasm3 currently has WASI support).

[radu-matei]

There have been a few improvements in the Wasmtime ecosystem over the last few months around startup time:

• the Wasmtime cache, which is now enabled for WAGI in Add option to use Wasmtime caching #19
• Wizer - this works great for modules with costly initializing functions.

It might make sense to add a readme section that talks about startup performance and some considerations.

[technosophos]

Oh, yeah... we totally should.

[technosophos]

@radu-matei Now that we have caching and preloading, I think we can close this one. Is that okay with you?

[radu-matei]

I agree, and even more with the instantiation improvements from Wasmtime.