Async I/O support and sans-io refactor (0.3.0)

[amitchone]

What's new

• AsyncThingSetTCP and AsyncThingSetUDPReceiver — asyncio-native RPC and report reception
• client.discover_schema() — returns a SchemaTree with by_id / by_path lookups; uses the 0x19 metadata overlay
• Gateway forwarding via target_eui kwarg (and -e CLI flag) — address CAN-side modules behind an IP gateway through a single TCP connection
• Decorated CLI output with names, types and friendly timeout hints
• UDP sniffer example with per-(IP, EUI) schema caching, EUI filtering and optional static record-field mapping
• Fixed sync TCP framing: accumulates across recv boundaries instead of assuming one response per call
  ## Breaking (pre-1.0)
  - ThingSet factory removed → instantiate ThingSetTCP / ThingSetCAN / ThingSetSerial directly
• ThingSetSock → ThingSetTCP - backends/ → transport/, ThingSetBackend → ThingSetTransport
• get_paths=True kwarg and 0x17 paths-overlay removed