Add JsString <-> char conversions

[RReverser]

These are pretty common and already supported via ABI conversions, yet pretty easy to get wrong when converting them manually.

Fixes #1363.

[alexcrichton]

Seems reasonable to me, thanks @RReverser! Just to confirm with a second opinion, @Pauan mind checking as well?

[Pauan]

Looks mostly good, just a couple things need to be fixed.

[RReverser]

@Pauan All your other suggestions make sense to me.

Any chance you could submit them using "Github suggestion" comments which I could easily accept (including from the phone)?

Otherwise I won't get around to this until after the Easter holiday (later next week).

[RReverser]

Ok fixed most nits (except the PartialEq thing, which I still think is useful in this context).

[alexcrichton]

I think I would agree with @Pauan here actually, could the From be removed as well as the PartialEq? They're compatible to add later but currently inconsistent with Rust's libstd

[RReverser]

Hmm, From is actually half of the PR... I don't think comparing with libstd makes sense, as it doesn't have string -> char conversion either, because, again, it has a separate type to represent it.

[Pauan]

Sorry for the delay, I recently caught a nasty cold.

I'm a bit wary of PartialEq, but I'm not strongly against it.

I feel like From is quite reasonable, so most of my concerns are with PartialEq.

I think they do seem rather useful, since JS itself doesn't make a distinction between strings and chars.

But I'm also okay with deferring them to a future PR.

[RReverser]

Sorry for the delay, I recently caught a nasty cold.

No worries and get well!

[RReverser]

Alright, I removed PartialEq<char> for now to avoid controversy around the PR and, indeed, we can still add it in the future.

[alexcrichton]

Thanks @RReverser!