6lowpan: rework compress/uncompress of addresses

This rework fix issues with a mac802154 layer address based ipv6
address.

This was never implemented before because the lowpan_uncompress_address
function has no information for the source address.

This rework is more familiar with the rfc 6282. It works no with a
prefcoung and postcoung information.

I added a new function to uncompress multicast addresses instead of
doing this in the same function like no multicast addresses.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>