Add serde support

[theotherphil]

ndarray has optional serde support, so hopefully we can just copy what they do.

[Andlon]

I think this is probably a good idea - however, would this mean that only rulinalg can read and write these serialized matrices? Perhaps then it's better to use a more standardized format (such as Matrix Market) and have individual functions for these, rather than using serde?

EDIT: It's probably a good idea in any case, because if a user has a type which contains a rulinalg matrix, the user would be able to automatically derive serde's Serialize/Deserialize, which may be convenient in many cases.