implement OptionIntoWasmAbi for JsValue

[ctaggart]

Motivation

On several occasions, I've want to map a parameter as Option<&JsValue> or a return type as Option<JsValue>, just to indicate explicitly that it is optional. I realize JsValue supports null & undefined.

Proposed Solution

Implement the trait.

Alternatives

I think of JsValue as the TypeScript any type. TypeScript also has the non-primitive object type that does not support null or undefined, but everything else. May be it would be better to add that so it makes more sense to have a Options<JsSome> or something like that.

[alexcrichton]

The reason we haven't implemented this is that there's no way to distinguish between Some(null) and None in JS, so there's no way to losslessly represent the value in JS. We've currently been requiring that Rust opts-in to .unwrap_or(...) or something like that.

[ctaggart]

Okay, I may revisit this again in the future with more concrete examples, after I ship the generator.