First try of serde implementation

[Stargateur]

About https://github.com/jaemk/cached/issues/49

This is a draft of a serde feature

1. This is a strait forward implementation
2. We could maybe do custom serialize and deserialize implementation to optimize the serialization and deserialization, for example store is not strictly needed, order could be serialize without next and prev, just a ordered vector.
3. This trust the input, this crate doesn't use unsafe so it's "ok" but that another reason to do a manual implementation.