Build burrow library into iOS and macOS app

This commit adds a build script, build-rust.sh, which compiles the
burrow crate from inside of Xcode. The network extension then links
against this crate.

burrow/Cargo.toml

@@ -3,7 +3,8 @@ name = "burrow"
 version = "0.1.0"
 edition = "2021"
 
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+[lib]
+crate-type = ["lib", "staticlib"]
 
 [dependencies]
 tokio = { version = "1.21", features = ["rt", "macros"] }

burrow/src/lib.rs

@@ -0,0 +1,3 @@
+pub fn hello_world() {
+    println!("Hello, world!");
+}

burrow/src/main.rs

@@ -1,7 +1,7 @@
 use tokio::io::Result;
 use tun::TunInterface;
 
-async fn lol() -> Result<()> {
+async fn try_main() -> Result<()> {
     let iface = TunInterface::new()?;
     println!("{:?}", iface.name());
 
@@ -10,5 +10,5 @@ async fn lol() -> Result<()> {
 
 #[tokio::main(flavor = "current_thread")]
 async fn main() {
-    lol().await.unwrap();
+    try_main().await.unwrap();
 }