WIP: Add serde support

[landaire]

This PR adds serde support gated behind the serde feature flag, allowing public structures this library provides to be serialized.

[landaire]

In this current state I don't think this PR should be merged as there's a little bit of style cleanup needed (some things don't need to be mut, etc.), docs, and tests should probably be added.

I wanted to get this PR submitted even in this state though to gauge interest in bringing this into project before making those changes. I saw in #5 that @mitsuhiko at least may have interest in this but it wasn't 100% clear to me.

[flier]

In this current state I don't think this PR should be merged as there's a little bit of style cleanup needed (some things don't need to be mut, etc.), docs, and tests should probably be added.

I wanted to get this PR submitted even in this state though to gauge interest in bringing this into project before making those changes. I saw in #5 that @mitsuhiko at least may have interest in this but it wasn't 100% clear to me.

Sure, remove the WIP in title when you ready :)