Wasm threading fixes

[PgBiel]

Currently implemented:

• Adds custom wasm binary name support proposed in WebAssembly support #438 (comment), sample usage:

  struct MyExtension;
  
  #[gdextension]
  unsafe impl ExtensionLibrary for MyExtension {
   fn override_wasm_binary() -> Option<&'static str> {
        // Binary name unchanged ("mycrate.wasm") without thread support.
        #[cfg(feature = "nothreads")]
        return None;
  
        // Tell gdext we add a custom suffix to the binary with thread support.
        // Please note that this is not needed if "mycrate.threads.wasm" is used.
        // (You could return `None` as well in that particular case.)
        #[cfg(not(feature = "nothreads"))]
        Some("mycrate-with-threads.wasm")
    }
  }

• Fixes wasm_nothreads compilation by not using compiled out function in async_runtime.rs (regression from Async Signals #1043)

• Fixes wasm_nothreads runtime errors on init due to thread locals being used for panic handling (regression from Panic handling: thread safety; set hook once and not repeatedly #1037)

TODO

Working on these, but feel free to review or merge what's already there.

• Potentially fix the rest of async_runtime to e.g. not use std::thread::current on nothreads builds (might need to be in a separate PR Let's do that in a separate PR (edit: using that function is actually fine, let's keep an eye on it in case anything else could be a problem)
• Automatically detect .threads.wasm when it still uses the crate name to avoid having to use the option introduced above outside atypical cases

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1093

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1093

[Bromeon]

Thanks a lot!

[PgBiel]

News:

• I've implemented a built-in check for .threads.wasm so the book instructions can be more easily followed. The wasm_binary option is now only used for non-standard names.
• I've added the requested docs, not sure if the example is too ideal given this, let me know if it should be changed or if it's ok for now

[Bromeon]

Looks mostly good, thanks!

[Bromeon]

Spontaneous thought, since wasm_binary is "dynamic" in nature (can be any string, not hardcoded identifier), did you consider making it a trait method of ExtensionLibrary rather than a proc-macro argument of #[gdextension]?

That way, it would be clearer to users that they can't just specify literals. Or would that complicate fetching the value?

[PgBiel]

Spontaneous thought, since wasm_binary is "dynamic" in nature (can be any string, not hardcoded identifier), did you consider making it a trait method of ExtensionLibrary rather than a proc-macro argument of #[gdextension]?

Nope, did not. Just changed it and seems to work nicely. Thanks for the suggestion!

I considered making it a const, but it didn't seem consistent with the rest, so I made it a method.

[PgBiel]

Been testing locally, worth mentioning. :)

[Bromeon]

Thank you!