Add serde behind a feature

Hi,

Whilst https://github.com/vleue/polyanya/issues/24 is open, it would be useful if polyanya's meshes could be serialised/deserialised because:

If using postcard (/other binary formats), it should be quicker to deserialize meshes from disk than generating and baking them at runtime
This would allow meshes to be preprocessed for use in bevy_pathmesh once Bevy's asset system v2 is merged

This PR just adds serde derives behind a feature gate accordingly.

I've raised another PR here for adding the same to the bvh2d crate.

vleue / polyanya