Wrap send and receive functions

[chrysn]

Currently, diondokter/nrf-modem#52 exposes just the RSSI function. This should get a minimal send and receive too, so that work on a MAC can start.

[chrysn]

The pull request has been updated to do minimal TX and RX. Lot of work left to expose them properly, though.

[chrysn]

Updated now to do TX with time precision and channel selection.

The next big thing is continuous RX (possibly interleved with RSSI); maybe mix in TX there right away, so that there can be a constant scheduling of RX-with-RSSI, TX-and-then-RX-again etc.