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.

Apple/Configuration/build-rust.sh

@@ -0,0 +1,69 @@
+#!/bin/bash
+
+export PATH="${PATH}:${HOME}/.cargo/bin:/opt/homebrew/bin:/usr/local/bin:/etc/profiles/per-user/${USER}/bin"
+
+if ! [[ -x "$(command -v cargo)" ]]; then
+    echo 'error: Unable to find cargo'
+    exit 127
+fi
+
+set -e
+
+cd -- "$(dirname -- "${BASH_SOURCE[0]}")"/../../burrow
+
+RUST_TARGETS=()
+
+IFS=' ' read -a BURROW_ARCHS <<< "${ARCHS[@]}"
+for ARCH in "${BURROW_ARCHS[@]}"; do
+    case $PLATFORM_NAME in
+        iphonesimulator)
+            case $ARCH in
+                arm64) RUST_TARGETS+=("aarch64-apple-ios-sim") ;;
+                x86_64) RUST_TARGETS+=("x86_64-apple-ios") ;;
+                *) echo "error: Unknown $PLATFORM_NAME arch, $ARCH"; exit 1 ;;
+            esac
+            ;;
+        iphoneos)
+            case $ARCH in
+                arm64) RUST_TARGETS+=("aarch64-apple-ios") ;;
+                *) echo "error: Unknown $PLATFORM_NAME arch, $ARCH"; exit 1 ;;
+            esac
+            ;;
+        macos*)
+            case $ARCH in
+                arm64) RUST_TARGETS+=("aarch64-apple-darwin") ;;
+                x86_64) RUST_TARGETS+=("x86_64-apple-darwin") ;;
+                *) echo "error: Unknown $PLATFORM_NAME arch, $ARCH"; exit 1 ;;
+            esac
+            ;;
+        *) echo "error: Unsupported platform $PLATFORM_NAME"; exit 1 ;;
+    esac
+done
+
+CARGO_ARGS=()
+for TARGET in "${RUST_TARGETS[@]}"; do
+    CARGO_ARGS+=("--target")
+    CARGO_ARGS+=("$TARGET")
+done
+
+CARGO_ARGS+=("--lib")
+
+if [[ $SWIFT_ACTIVE_COMPILATION_CONDITIONS == *DEBUG* ]]; then
+    CARGO_DIR="debug"
+else
+    CARGO_ARGS+=("--release")
+    CARGO_DIR="release"
+fi
+
+if [[ -x "$(command -v rustup)" ]]; then
+    CARGO_PATH="$(dirname $(rustup which cargo)):/usr/bin"
+else
+    CARGO_PATH="$(dirname $(readlink -f $(which cargo))):/usr/bin"
+fi
+
+env -i PATH="$CARGO_PATH" cargo build "${CARGO_ARGS[@]}"
+
+mkdir -p "${BUILT_PRODUCTS_DIR}"
+/usr/bin/xcrun --sdk $PLATFORM_NAME lipo \
+    -create $(printf "${PROJECT_DIR}/../target/%q/${CARGO_DIR}/libburrow.a " "${RUST_TARGETS[@]}") \
+    -output "${BUILT_PRODUCTS_DIR}/libburrow.a"