CI: Added caching of Rust stuff to potentially speed up action (#129)

.github/workflows/rust.yml

@@ -11,12 +11,15 @@ env:
 
 jobs:
   build:
-
     runs-on: ubuntu-latest
 
     steps:
     - uses: actions/checkout@v2
+
+    - uses: Swatinem/rust-cache@v1
+
     - name: Build
       run: cargo build --verbose
+
     - name: Run tests
       run: cargo test --verbose