Compressed packet support

[sh123]

Hello,

To maximize chance of packet being decoded it would be nice to have support for compressed packet format as specified in Chapter 9 in APRS specificaion, it has fixed 13 byte length, which will increase probability of being correctly decoded in weak or interference conditions.

[markqvist]

Thanks for the suggestion, that should definitely be added. I'm putting most of my time into RNS at the moment, but I'll get around to this eventually :)