[codec] add decoding for allowlists that are passed back from the NCP (#125)
[codec] add decoding for allowlists that are passed back from the NCP (#125)

Add unpacking for the various types of responses that come back from
the MAC_ALLOWLIST property. Note that there are three different
formats, so we try them from the most complex/involved to the least
complex/involved.

1. Try to decode an array of structures composed of an EUI64 and an
   unsigned 8-bit integer (from querying the value)
2. If that fails, try to decode a single entry composed of an EUI64
   and an unsigned 8-bit integer (from inserting a value)
3. If that fails, try to decode a single entry composed of only an
   EUI64 (from deleting an entry)
4. If all of these fail, return None
openthread/pyspinel,openthread/pyspinel