In old lib code:
* pass header all the way down to device
* have device send header using the same code it uses to send payload
* have device treat received header as identical to payload
* use new MessageHeader in existing app (make sure it is packed properly)

In the sudomesh code:
* move this rf95 lib into the layer2 project
* make RadioInterface the new layer one API (move over set radio options)
* change meshtastic app to use new layer one API

Now meshtastic is sharing layer one with disaster radio.
* change mesthastic app to use new layer two API (make sure broadcast still works for max TTL of 1)

Now meshtastic is sharing layer two with disaster radio.

* make simulation code work with new API
* make disaster radio app work with new API 

later:
* implement naive flooding in the layer2 lib, use TTL limit max depth of broadcast
* allow packets to be filtered at the device level RX time based on dest addr (to avoid waking main CPU unnecessarily)