hashmap: fix schema for current syntax & update links and descriptions of impls

[rvagg]

Ref: #26

1. type Foo list [ Thing] got shortened to type Foo [ Thing ] in schemas.
2. We removed the ability to do field modifiers on anything but map representation structs, so the tuple here with an implicit isn't allowed. This matches the current implementation I think https://github.com/rvagg/iamap/blob/fad95295b013c8b4f0faac6dd5d9be175f6e606c/iamap.js#L117-L119, but it would be nice to be able to say that the last field in a tuple representation struct can have an implicit, so that's a thing we could consider in a future iteration.

Mikeal also mentioned that the current JavaScript implementation via iamap doesn't match the schema, I'm not sure where exactly but I need to go through and figure that out. That's still a manual process while we lack solid tooling.

I've also updated the implementation links and descriptions. I didn't realise ipld-hashmap wasn't on the list. It's much more pleasant to use than IAMap directly.

Thanks @chafey for the heads-up.