nib: New module to add neighbor cache access

[chrysn]

As suggested by @mcr.

This is incomplete: It does not yet expose the various aspects of an entry's state.

In practice, this could be used by a new riot-coap-handler-demos variant for output like this:

$ aiocoap-client coap://'[fe80::3c63:beff:fe85:ca96%tapbr0]'/nib/neigh
# CBOR message shown in Diagnostic Notation
[_
    [
        6,
        h'862e80417409',
        IP'fe8::f30:40e4:6c93:e17d',
        "reachable",
        "GC",
    ],
]