Add code coverage back to CI.

.github/workflows/ci.yml

@@ -234,4 +234,46 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: clippy
-          args: --all-features --bins --examples --tests --lib
+          args: --all-features --bins --examples --tests --lib
+
+  coveralls_io:
+    needs: [detailed_tests]
+    name: Coverage
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: ⚡ Cache
+        uses: actions/cache@v2
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            target
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Install toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+          override: true
+          default: true
+
+      - name: Install cargo-tarpaulin
+        uses: actions-rs/install@v0.1
+        with:
+          crate: cargo-tarpaulin
+          version: latest
+          use-tool-cache: true
+
+      - name: Coverage Report with tarpaulin
+        uses: actions-rs/cargo@v1
+        with:
+          command: tarpaulin
+          args: --all --all-features --out Lcov -- --test-threads 1
+
+      - name: Upload Coverage
+        uses: coverallsapp/github-action@master
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          path-to-lcov: ./lcov.info