keytree: serialize Xpub/Xprv as 64-byte strings

[oleganza]

• [x] Add custom serde::{Serialize,Deserialize} impls.
• [ ] If serializer is human-readable, encode as hex string. (See hex-buffer-serde crate as an example, but do not use it as dependency, since we want to apply encoding to the whole container, not to a single field).
• [x] Put impl in a separate module serialization.rs.

[oleganza]

Done in #346