Enable Wasm weak references for automatic garbage collection

[cycraig]

Description of change

Enable Weak References in the Wasm bindings. This ensures that Wasm memory is freed automatically by the garbage collector, otherwise it will continue to grow unless the developer explicitly calls .free() on every exported instance or the instance is passed back to a function which takes ownership of it, allowing Rust to free it.

Unfortunately wasm-pack does not support passing the --weak-refs flag to wasm-bindgen (issue), so we have to invoke cargo, wasm-bindgen, and wasm-opt manually.

Another downside is that wasm-bindgen currently has a bug where parameters passed by value fail to deregister Finalizer handlers. This causes runtime errors from automatic garbage collection trying to free the instance when it is no longer referenced from the Javascript side, since WasmRefCell sees the instance as still borrowed.
See: rustwasm/wasm-bindgen#2677

Fortunately this is easy-enough to circumvent by borrowing and cloning owned parameters instead. I added a build-time lint to prevent this problem going forward until the above PR is merged.

Type of change

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Wasm tests and examples pass locally. I checked the memory usage locally with Chrome dev tools to confirm that the memory is freed by automatic garbage collection now.

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[PhilippGackstatter]

Looks good to me! Good idea with the lint, too.

[eike-hass]

Since we don't use wasm-pack anymore, I assume this can be removed.

[cycraig]

We still invoke wasm-pack for running the unit tests (it's more complicated to set up that environment apparently).

[eike-hass]

Other then the two minor points, LGTM! Will accept now already.
Let me know when it is ready, then we can admin merge (because of the windows runner issue).

[yisibl]

Starting with v0.2.91 "wasm-bindgen does use the TC39 weak references proposal if support is detected. At the time of this writing all major browsers do support it."