Add serialization support

I'm currently converting [my simulation](https://github.com/syberant/evolvim) to using nphysics2d (thanks a lot by the way) but that unfortunately does mean that I can no longer easily `Serialize` and `Deserialize` everything with serde. There were some interesting things happening in #184 but it seems to be a bit quiet now. My ultimate goal is to be able to use `Serialize` and `Deserialize` with `World` and `BodyHandle` but small steps are probably a good idea.

I would like to get my hands dirty and do it or part of it myself but thought I'd ask for some advice here first. I currently have close to no knowledge of the insides of this crate and can not anticipate the challenges ahead so some advice will be much appreciated! One thing I do anticipate however is ncollide also not having serialization support, should I start there first?