Provide constant time implementations of PartialEq for private values

[BobWall23]

Implement PartialEq.eq for PrivateKey, Plaintext, DerivedSymmetricKey using a constant time function.

AC:
[ ] These three types all have PartialEq.eq implementations.
[ ] The implementations are data-invariant / constant time - the evaluation of the equality between two values of the same type will always execute the same number of operations, regardless of the contents of either value.