0.5.0-alpha.1

Serial TNCs, and a messaging engine for AprsMap's transmit phase. KISS TNCs on USB serial (#160), proven on hardware, which AprsMap 0.5.0 (#177) needs; and AprsNet.Messaging (#189): message IDs, retries, acks, reply-acks and bulletins, proven live with aprscli. Each capability is a parent issue labelled capability; its sub-issues are the work. Out of scope: beaconing and queries (G11, G19), GPS (G17, G30, #127), and the registered TOCALL (#155, #161), which ships in a release after APNETL is assigned.