Add support for basic hash/object conversions

[superstator]

Addresses #169

[simlay]

Hi there! Thanks for the contribution!

At first glance, I think this is pretty good. I've got a few nits.

• Add to the CHANGELOG.md.
• Remove or make add more of a description to the README.md in the hash example.

I don't wanna expand the scope of this task too much but I wonder if we care about impl<'a, T> JSValue<'a> for HashMap<U, T> where U: Eq + Hash + JSValue, T: JSValue (and TryIntoJS). Thoughts?

[superstator]

Ha, I used the array example as a template and obviously missed some cruft. I will update the readme and changelog, and give the additional impl some thought.

[superstator]

I played around a little with the idea of a generic hash key type, but I think it would effectively mean needing to support Map() on the JS side, which is not a slam dunk via NAPI. Doable, but maybe best left until somebody has a strong need for it.

[simlay]

LGTM! Thanks!

[digikata]

LGTM

[simlay]

bors r+

[sehz]

bors r+

[yannleretaille]

Was there a reason this wasn't merged? It'd be very useful to have native support for HashMap.