During the review of #9485 [we found out][1] that an assertion in this function was invalid. However, the documentation on this assertion wasn't removed on that. This fixes that. [1] https://github.com/RIOT-OS/RIOT/pull/9485#issuecomment-407794280
This refactors sending/encoding part of `gnrc_sixlowpan_iphc` to the more layered approach modeled in #8511. Since the reception part is already was pretty complicated to refactor, I decided to divide send and receive up into separate changes.
This abstracts the sending and receiving of 6Lo packets to the new 6Lo sub-layer model introduced in #8511 and exemplifies it as well.