Generic `Session`

[cortopy]

A Session has a data field of type HashMap<String, String>. This forces a session to always use JSON for serialization/deserialization.

In my case I would prefer if it was a HashMap<String, Bytes> so that I can use more efficient serde methods (e.g.: bincode, which this crate already uses)

Would it be possible to change Session so that it becomes generic like Session<T> so that data becomes HashMap<String, T>?