Setup caching for CI

Automatically caches compiled dependencies and - more importantly in this case - the cargo
registry. Downloading the registry currently takes up the majority of CI time.

Author: LingMan

.github/workflows/main.yml

@@ -30,6 +30,8 @@ jobs:
         uses: maxim-lobanov/setup-xcode@v1
         with:
           xcode-version: '13'
+      - name: Setup cache
+        uses: Swatinem/rust-cache@v2
       - name: Test (no features)
         run: cargo test --no-default-features
       - name: Test (all features)