core: Remove data packing section

[darconeous]

This issue is tracking the removal of the data-packing format section of spinel-core, instead using more IETF-friendly notation wherever byte packing is referenced.

This will help avoid raising too many eyebrows, in addition to preventing people from getting the impression that the data-packing is actually a core part of the protocol rather than a convenient shorthand notation.

Note that this notation will still continue to be used in OpenThread's spinel parser, since in that case it is a practical benefit.