Fixed missing wipes

Fixed missing wipes

Found 4 internal buffers that were not properly wiped.  While this were
unlikely to turn into an exploitable vulnerability, we can never be too
cautious.

- XChacha20 initialisation failed to wipe its intermediate key.
- Blake2b initialisation failed to wipe a copy of its secret key.
- EdDSA failed to wipe an internal field element
- EdDSA failed to wipe an internal group element