Upgrade tarpaulin to 0.21.0 (#54)

Upgrade `tarpaulin` to 0.21.0 to work around what looks like a bug in coverage report.

Fixes #55

.github/workflows/ci.yaml

@@ -100,7 +100,7 @@ jobs:
         if: runner.os == 'linux'
       - name: Install cargo-tarpaulin
         run: |
-          RUST_BACKTRACE=1 cargo install --version 0.19.1 cargo-tarpaulin
+          RUST_BACKTRACE=1 cargo install --version 0.21.0 cargo-tarpaulin
       - name: Generate code coverage
         run: |
           RUST_BACKTRACE=1 cargo tarpaulin --all-features --verbose --timeout 120 --out Lcov --workspace