Normalize Argument Names

This commit normalizes argument names, so that they are uniform across both the low-level and high-level APIs.  Additionally, relevant object properties and named tuple elements have been updated as well.

- usage, cred_usage --> usage
- lifetime, ttl --> lifetime
- desired_foo --> foo (excpet desired_output_size)
- mech_type, mech --> mech (plurals updated as well)