Implement 6lowpan header compression

This implements 6lowpan header compression per rfc 6282. While the
decompression logic is full-fledged (to accept almost any 6lowpan
compressed packet on ingress), the compression logic does *not* actually
compress the packet.

Fully fledged compression might be submitted in a follow up, to make
integration testing possible.

Test: TH
Change-Id: I5ea98a06e502fe0752cfe827dbdf8ee4a7dfb24c
4 files changed