Refactor MechanismWithParam

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 https://github.com/danni/python-pkcs11/pull/97#discussion_r502974498 for more information.

Start by locating MechanismWithParam, mechanism_param, and the current mechanism handling, then read the discussion on pull request 97. Define how Mechanism accepts CK_MECHANISM or a Mechanism instance and how typed subclasses allocate PKCS#11 parameter structures. Done means the old mechanism_param approach is removed and the new extensible API supports the described default and subclass forms.