Simplify verification and secure_cmp interfaces

[brycx]

As pointed out in issue #92 raised by @snsmac, both verify() and secure_cmp currently return Result<Ok(bool), UknownCryptoError>.

Returning a bool is however redundant, as the verification or comparison only will return Ok() if it was successfull and UknownCryptoError if not. If a user needed a bool to represent the result, this could be done with .is_ok()/.is_err().

Because of this, it seems it would make sense to change secure_cmp and all verify() interfaces in orion to return Result<(), UknownCryptoError>, dropping the needless bool.

Before doing so, these new interfaces should be tested in orion-dudect for constant-time execution.

[brycx]

Testing on different platforms has not shown any issues with constant-time execution, when switching to the verification described above.