icmpv6: put message definitions in their own files

sys/include/net/icmpv6.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_icmpv6/types.h

sys/include/net/ng_ndp/types.h

sys/include/net/ng_rpl.h

sys/net/network_layer/ng_icmpv6/echo/ng_icmpv6_echo.c

sys/net/network_layer/ng_icmpv6/ng_icmpv6.c

sys/net/network_layer/ng_ndp/internal/ng_ndp_internal.c

sys/net/network_layer/ng_ndp/ng_ndp.c

sys/net/routing/ng_rpl/ng_rpl.c

sys/net/routing/ng_rpl/ng_rpl_control_messages.c

sys/shell/commands/sc_icmpv6_echo.c