Add top-level type for station ID

[cbs228]

(Continued from discussion in #50)

We should add a new data type to represent Station ID: callsign + SSID. Notional requirements:

• Canonicalize the string representation

  • n0call → N0CALL
  • N0CALL-0 → N0CALL
  • N0CALL- → N0CALL

• Accept only callsigns 3 – 7 ASCII characters

• Accept only SSIDs in A-Z and numeric 1 to 15

• Conversion to/from 6-byte wireline representation

• Comparison function (i.e., is this packet for me?)

The data type should probably contain:

1. The ASCII C string representation
2. The compressed 6-byte wireline representation

Everything except for (2) is strictly optional, but it is handy to have a type that can be snprintf()'d without further transforms.

Construction should probably involve a round-trip through the compressor to ensure it can be transmitted losslessly.

NOTE: for future protocol designs, I favor the 28-bit representation from js8call, which seems to do a good job of representing all the world's callsigns. We cannot change the ARDOP wireline representation at this time, but I would encourage future protocols to adopt a more complete encoding.

[pflarue]

PR #79 implements an improved system for handling station IDs.