Refactor MechanismWithParam

[danni]

MechanismWithParam was a hack to allow for various param mechanisms, but it doesn't scale.

mechanism_param should be removed and mechanism be replaced to allow either a CK_MECHANISM or a Mechanism class.

The default Mechanism class would be mechanism=Mechanism(CK_MECHANISM, param=...) passing mechanism=CK_MECHANISM would be equivalent to mechanism=Mechanism(CK_MECHANISM).

The Mechanism class could be inherited to provide for specific, typed mechanisms Mechanism.RSA_PKCS_PSS(params...) these would be responsible for allocating their appropriate structure in PKCS#11 and passing it as the mechanism parameter.

This is both more Pythonic, typeable (when typing is added to python-pkcs11) and allows for extension into more mechanism types, including extension outside of python-pkcs11.

See the discussion in #97 (comment) for more information.