nib: implement public NIB functions up to link-local AR

Makefile.dep

sys/include/net/gnrc/ipv6/nib.h

sys/net/gnrc/network_layer/ipv6/nib/_nib-6ln.c

sys/net/gnrc/network_layer/ipv6/nib/_nib-6ln.h

sys/net/gnrc/network_layer/ipv6/nib/_nib-6lr.c

sys/net/gnrc/network_layer/ipv6/nib/_nib-6lr.h

sys/net/gnrc/network_layer/ipv6/nib/_nib-arsm.c

sys/net/gnrc/network_layer/ipv6/nib/_nib-arsm.h

sys/net/gnrc/network_layer/ipv6/nib/_nib-internal.c

sys/net/gnrc/network_layer/ipv6/nib/_nib-internal.h

sys/net/gnrc/network_layer/ipv6/nib/nib.c

tests/unittests/tests-gnrc_ipv6_nib/tests-gnrc_ipv6_nib-nc.c