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.

.github/workflows/build-rust.yml

@@ -50,7 +50,7 @@ jobs:
       shell: bash
       run: sudo apt-get install -y ${{ join(matrix.packages, ' ') }}
     - name: Install Rust
-      uses: dtolnay/rust-toolchain@master
+      uses: dtolnay/rust-toolchain@stable
       with:
         toolchain: stable
         components: rustfmt