Add TLS transport support

Connect over TLS (default port 6697) using native-tls with the system
OpenSSL trust store for certificate verification. Unify the plaintext and
TLS transports behind a single Box<dyn Read + Write>, dropping the
read/write socket clone that a TLS session cannot support. Add a `tls`
config key and IRC_TLS override; the port auto-defaults to 6697 when TLS
is enabled.

Cargo.toml

@@ -10,9 +10,12 @@ authors = ["a.srenov"]
 name = "rustbot"
 path = "src/main.rs"
 
-# Intentionally dependency-free. The IRC protocol layer lives in `src/irc.rs`
-# and follows the modern IRC client spec: https://modern.ircdocs.horse/
+# The IRC protocol layer is hand-rolled in `src/irc.rs` (modern IRC client
+# spec: https://modern.ircdocs.horse/). The sole dependency is native-tls for
+# the optional TLS transport; it wraps the system OpenSSL, so TLS security
+# fixes arrive through normal OS updates rather than a vendored crypto crate.
 [dependencies]
+native-tls = "0.2"
 
 [profile.release]
 lto = true