feat: implement RNS transport (Smol Mail 1.2) over microReticulum

Cargo.toml

@@ -8,6 +8,11 @@ description = "Smol Mail server"
 name = "bunshin"
 path = "src/main.rs"
 
+[features]
+# RNS carrier over microReticulum (RNS.md); needs MICRORETICULUM_SOURCE_DIR
+# at build time, provided by the flake.
+rns = ["dep:cc"]
+
 [dependencies]
 snow = "0.9"
 rusqlite = { version = "0.32", features = ["bundled"] }
@@ -20,3 +25,6 @@ clap = { version = "4", features = ["derive"] }
 log = "0.4"
 env_logger = "0.11"
 anyhow = "1"
+
+[build-dependencies]
+cc = { version = "1", optional = true }