Use specs' saveload functionality for serialization

Supposedly [`saveload`](https://docs.rs/specs/0.12.1/specs/saveload/index.html) is the "right" way of handling serialization/deserialization of component data with specs. The documentation doesn't make it very clear how to use this functionality, but [there's an example](https://github.com/slide-rs/specs/blob/master/examples/saveload.rs) that should be enough to get us started.

This change will likely change the format of the serialized data, so we'll have to update the [Electron editor](https://github.com/randomPoison/amethyst-editor) to work with the new format.