Changed dependency err-impl to be a git dependency.

Re-enabled workflow for `clippy` and testing.

.github/workflows/rust.yml

@@ -12,3 +12,7 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2
+      - name: Clippy
+        run: cargo clippy --verbose
+      - name: Run tests
+        run: cargo test --verbose