Change c_create_mech_list to c_create_oid_set

The function `c_create_mech_list` in the low-level converters
really just converted a OID set struct into a list of OIDs.
Ergo, it should be named as such.

Additionally, it makes more sense for the return value to be
a set, so make it as such.
Implement inquire_names_for_mech in low-level API

This commit implements the inquire_names_for_mech method
from RFC 2744, and adds a simple test as well.

Part of #10
Implement inquire_mechs_for_name in low-level API

This commit implements the inquire_mechs_for_name method
from RFC 2744, and adds a simple test as well.

Closes #10