Generate bindings for indexed struct properties

[RReverser]

This allows to export fields of tuple-like structs as indexed JS properties.

While we could rename it to _0, $0 or something else, this feels like the most natural mapping of "indexed" properties to JavaScript concepts, and just works out of the box with current JS and TypeScript codegens.

[alexcrichton]

Seems reasonable to me, thanks @RReverser! Could you ensure that this is documented in the reference as well?

[RReverser]

Ah yes. I'm leaving for the Easter holiday now but can back to it afterwards.

[RReverser]

@alexcrichton Hmm, I might need help figuring out where best to put it. There currently doesn't seem to be a dedicated section describing how named properties are bound on the JS side? (I've found only exported_types.rs, but it doesn't demonstrate properties usage, only the class itself)

[alexcrichton]

Ah that's true yeah, mind starting a page which describes the JS api created for exported rust structures? It could go in the area for attributes on exports perhaps in the reference

[RReverser]

I'm thinking maybe just adding some property usage to the same existing exported-rust-types.md + exported_types.rs + exported_types.js?

[RReverser]

I've added property examples to files mentioned above, let me know how it looks.

[alexcrichton]

Thanks!