Move pwd grp functions to pwdgrp module; require binary fields

Move pwd grp functions to pwdgrp module; require binary fields

Move the pwd and grp related functions in helpers to a new more
focused pwdgrp module and adjust them (for py3) to require and provide
binary (bytes) values for the appropriate fields: name, passwd, etc.
These fields are arbitrary bytes with no encoding information on
platforms like Linux and *BSD, so must be handled that way.

Create our own Passwd and Group classes to carry the data, and make
them __slots__ based since they're quite a lot smaller, even than
namedtuples:
https://justmachinelearning.com/2019/07/29/python-consumes-a-lot-of-memory-or-how-to-reduce-the-size-of-objects/

See the DESIGN section on Python 3 strings for additional information.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>