Implement Zeroize for secret types

[leonbotros]

• implement/derive zeroize for (copies of) user secret keys,
• implement/derive zeroize for (copies of) master secret keys,
• implement/derive zeroize for (copies of) shared secrets.
• also add the trait bounds to the KEM/IBE traits.

[leonbotros]

Currently, all these types are Copy, which means we cannot implement ZeroizeOnDrop. Still this enhancement would allow for users to manually zeroize.