ipv6: move non-GNRC types and functions in their own modules

Makefile.dep

sys/Makefile

sys/include/net/ipv6.h

sys/include/net/ipv6/ext.h

sys/include/net/ipv6/ext/rh.h

sys/include/net/ipv6/hdr.h

sys/include/net/ng_icmpv6.h

sys/include/net/ng_icmpv6/echo.h

sys/include/net/ng_icmpv6/error.h

sys/include/net/ng_ipv6/ext.h

sys/include/net/ng_ipv6/hdr.h

sys/include/net/ng_ndp.h

sys/include/net/ng_ndp/internal.h

sys/include/net/ng_rpl/srh.h

sys/include/net/protnum.h

sys/net/crosslayer/ng_pktdump/ng_pktdump.c

sys/net/link_layer/ng_netdev_eth/ng_netdev_eth.c

sys/net/link_layer/ng_slip/ng_slip.c

sys/net/network_layer/ipv6/ext/rh/Makefile

sys/net/network_layer/ipv6/ext/rh/ipv6_ext_rh.c