Flexible traits

[leonbotros]

This PR aims to solve #6 . When some schemes require a public key in the key generation or decryption/decapsulation, an operation may panic. Instead, the code now won't compile. This change is breaking, so we should consider carefully.

This approach has several drawbacks:

• when the concrete type of IBKEM is unknown, you have to have both the parameters to perform extraction and decryption.
• it is a breaking change of our public API,
• it bumps the MSRV to 1.65 (since we use GATs in the solution).

Another approach is to put the pk in the sk and usk, respectively. But imo, this does not make sense from a practical point of view:

• In the case of extraction, you would expect the PKG to have both the mpk and msk.
• In the decryption, a client that decrypts is also often a client that also encrypts, so you would expect the client to have the public key in its cache.