Add a JSON export API

[rnewman]

It would be useful for debugging and testing to be able to dump an entire database as JSON.

This should be relatively easy to implement: all of our types work with Serde, so we would 'just' need to make an iterator over keys and values serialize as a container.

[rnewman]

N.B., implemented correctly, this actually gives us serialization to an open set of possible formats (e.g., CBOR). Hooray Serde!

[victorporof]

This will be fixed by https://github.com/mozilla/rkv/pull/158 in its final form.