Upgrades to zig 0.9 and more

[trashhalo]

I was using this library as a foundation for https://github.com/trashhalo/create-zig-wasm-app
Here are the changes I made a long the way. Let me know if you want me to try and cut this up to be smaller.

1. zig 0.9 support. allocator changes mostly
2. be more forgiving when functions return implied error union types. (Function takes anyerror now)
3. allow functions to not return anything. Function now is optional. send a 0 len pointer back to client in this case
4. switch some things to const to allow inlining the function call loop
5. when a function errors serialize the name back to the client so you can get an idea of what went wrong
6. using an arraylist instead of doing len math to alloc the exact right number of bytes.
7. give caller control over allocator creation