Add ICMPv6 Destination Unreachable and TUN reader/writer architecture

- New icmp.rs module: builds RFC 4443 compliant ICMPv6 Type 1 Code 0
  responses with proper checksum and packet validation
- TUN reader/writer separation: fd duplication allows independent I/O
  on separate threads
- TunWriter services mpsc queue for multiple future packet sources
- Reader now sends ICMPv6 errors for unroutable packets instead of
  silently dropping them
- 171 tests passing

Cargo.toml

@@ -16,6 +16,7 @@ hex = "0.4"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 tun = { version = "0.7", features = ["async"] }
+libc = "0.2"
 rtnetlink = "0.14"
 tokio = { version = "1", features = ["rt", "macros", "signal", "sync"] }
 futures = "0.3"