Add codecov ci workflow

Author: notmandatory

.github/workflows/code_coverage.yml

@@ -0,0 +1,61 @@
+on: [push, pull_request]
+
+name: Code Coverage
+
+jobs:
+  Codecov:
+    name: Code Coverage
+    runs-on: ubuntu-latest
+    env:
+      RUSTFLAGS: "-Cinstrument-coverage"
+      RUSTDOCFLAGS: "-Cinstrument-coverage"
+      LLVM_PROFILE_FILE: "report-%p-%m.profraw"
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Install lcov tools
+        run: sudo apt-get install lcov -y
+      - name: Install rustup
+        run: curl https://sh.rustup.rs -sSf | sh -s -- -y
+      - name: Set default toolchain
+        run: rustup default nightly
+      - name: Set profile
+        run: rustup set profile minimal
+      - name: Add llvm tools
+        run: rustup component add llvm-tools-preview
+      - name: Update toolchain
+        run: rustup update
+      - name: Cache cargo
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/bin/
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+      - name: Install grcov
+        run: if [[ ! -e ~/.cargo/bin/grcov ]]; then cargo install grcov; fi
+      - name: Test
+        # WARNING: this is not testing the following features: test-esplora, test-hardware-signer, async-interface
+        # This is because some of our features are mutually exclusive, and generating various reports and
+        # merging them doesn't seem to be working very well.
+        # For more info, see:
+        # - https://github.com/bitcoindevkit/bdk/issues/696
+        # - https://github.com/bitcoindevkit/bdk/pull/748#issuecomment-1242721040
+        run: cargo test
+      - name: Run grcov
+        run: mkdir coverage; grcov . --binary-path ./target/debug/ -s . -t lcov --branch --ignore-not-existing --ignore '/*' -o ./coverage/lcov.info
+      - name: Generate HTML coverage report
+        run: genhtml -o coverage-report.html ./coverage/lcov.info
+
+      - name: Coveralls upload
+        uses: coverallsapp/github-action@master
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+      - name: Upload artifact
+        uses: actions/upload-artifact@v2
+        with:
+          name: coverage-report
+          path: coverage-report.html