Implement To/FromWasmAbi for arrays

[Jules-Bertholet]

Allows using Rust arrays as arguments and return values for wasm-bindgen functions.

[Jules-Bertholet]

This is not ideal. I see a few alternatives here:

1. Accept the panic
2. Add a T: Default bound and fill missing values with default()
3. Add a T: JsCast bound and fill missing values with JsValue::UNDEFINED

[Jules-Bertholet]

I've gone ahead and implemented alternative 2

[alexcrichton]

Thanks for the PR! I'd prefer to not implement this via this strategy though becuase this can be a hidden performance trap relative to allocations and transferring/copying memory. I don't necessarily know of an alternative way to implement this that doesn't suffer from such an issue, but I would recomend consumers instead work with Vec<T> and then convert to [T; N] themselves (e.g. with the array-init crate you found) instead of baking that directly into wasm-bindgen