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"] }