Implement Zeroize for Private structs on presign Module

[gatoWololo]

We will start with ensuring the Private structs inside the presign module are properly zeroized. We pick this module since there has been previous work on this #511.

• See Epic Implement Proper Zeroization for Sensitive and Secret Data #517 for details on how this work should be done.
• The Secret* types detailed in Implement Proper Zeroization for Sensitive and Secret Data #517 do not exists yet. This Issue will be additional work as we must implement these types and their associated functionality.