fix: include doctests in test coverage calculations (#397)

* update tarpaulin used in CI to latest available version

* fix: try including `--doc --examples` in tarpaulin arguments, to include coverage by doctests and code examples

* revert back to 0.22.0 until tarpaulin action is fixed: actions-rs/tarpaulin#23

It seems like the download is broken in the `actions-rs/tarpaulin` GitHub Action for all tarpaulin versions above `0.22.0`. See this pending fix for details:
actions-rs/tarpaulin#23

* try with the nightly toolchain for code coverage of doctests

* document in-code the current problem with artefact download blocking tarpaulin update

* try with `--run-types Tests,Doctests` for full coverage: xd009642/tarpaulin#538

.github/workflows/rust.yml

@@ -52,10 +52,10 @@ jobs:
         with:
           submodules: recursive
 
-      - name: Install stable toolchain
+      - name: Install nightly toolchain
         uses: actions-rs/[email protected]
         with:
-          toolchain: stable
+          toolchain: nightly
           override: true
 
       - name: Install system dependencies
@@ -67,8 +67,9 @@ jobs:
       - name: Run cargo-tarpaulin
         uses: actions-rs/[email protected]
         with:
+          # TODO: update to latest tarpaulin once artefact download is fixed: https://github.com/actions-rs/tarpaulin/pull/23
           version: "0.22.0"
-          args: "--all-features --out Lcov -- --test-threads 1"
+          args: "--all-features --run-types Tests,Doctests --out Lcov -- --test-threads 1"
 
       - name: Upload coverage
         uses: coverallsapp/github-action@v1