Currently the bitfield type mixes up the order of bits: While the byte
order is big-endian (most significant byte first), the bit order of each
single byte within the bitfield is little-endian (most significant bit
last). While this isn't a problem for most applications of the bitfield
type it becomes one when sending the bitfield over the network (as done
e.g. in the [ACKs of Selective Fragment Recovery][SFR-ACKs]).
This change unifies byte order and bit order to both be most
significant bX first.
[SFR-ACKs]: https://tools.ietf.org/html/draft-ietf-6lo-fragment-recovery-07
