Public key serilzation

[thesimplekid]

Public key serialization/deserialization seems to be broken.

• [ ] Tests for pubkey
• [ ] Test for seckey

[thesimplekid]

https://github.com/thesimplekid/cashu-crab/commit/fd955f22cb15a0b08ef8b7165e83b85e3ab68789

Deserialization for Secret key is removed as it is not needed at this point, but should be implemented later.