Refactor AX.25/APRS code out of afsk module

Move AX.25 and APRS code out of the afsk.py module, leaving
only Bell-202 encoding code in `afsk`. Move the main() entry
point to ax25.py since it's using both the UI class and AFSK
encoding, and the afsk module no longer depends on the AX.25
code.

Rename the main afsk public interface to `encode` rather than
`afsk`.

Rename binary entry point to `aprs` rather than `afsk`, since
we're encoding AX.25 UI packets suitable for APRS use rather
than raw AFSK data from binary.