Support for Weak References

By default 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.

Without weak references your JS integration may be susceptible to memory leaks in Rust, for example:

• You could forget to call .free() on a JS object, leaving the Rust memory allocated.
• Rust closures converted to JS values (the Closure type) may not be executed and cleaned up.
• Rust closures have Closure::{into_js_value,forget} methods which explicitly do not free the underlying memory.

These issues are all solved with the weak references proposal in JS. FinalizationRegistry will ensure that all memory is cleaned up, regardless of whether it's explicitly deallocated or not. Note that explicit deallocation is always a possibility and supported, but if it's not called then memory will still be automatically deallocated if FinalizationRegistry support is detected.