Implement serde derives replacement

[jam1garner]

Currently the easiest way to serialize/deserialize modular-bitfields using serde is by converting to/from another struct when serializing/deserializing. Ideally this could be handled automatically by modular-bitfield similarly to Debug impls (see #32).

I'd be willing to handle implementation, however I'd appreciate thoughts on if this is an acceptable feature for inclusion and if so any thoughts on how you like it to be implemented.