rework github actions for code coverage

This reworks our GitHub Actions workflow to include code coverage via
tarpaulin. Note that this is essentially directly lifted from the
again[1] crate's methodology.

Fixes apache#164.

[1]
https://github.com/softprops/again/blob/dd5f0013533e28f803b282ebc281e9525ca64d86/.github/workflows/main.yml

Author: maxcountryman

.github/workflows/rust.yml

@@ -3,28 +3,58 @@ name: Rust
 on: [push]
 
 jobs:
-  build:
+
+  codestyle:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up Rust
+        uses: hecrj/setup-rust-action@v1
+        with:
+          components: rustfmt
+          rust-version: nightly
+      - uses: actions/checkout@v2
+      - run: cargo fmt -- --check --config-path <(echo 'license_template_path = "HEADER"')
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up Rust
+        uses: hecrj/setup-rust-action@v1
+        with:
+          components: clippy
+      - uses: actions/checkout@v2
+      - run: cargo clippy --all-targets -- -D clippy::all
+
+  compile:
     runs-on: ubuntu-latest
+    steps:
+      - name: Set up Rust
+        uses: hecrj/setup-rust-action@v1
+      - uses: actions/checkout@master
+      - run: cargo check --all
 
+  test:
+    needs: [codestyle, lint, compile]
+    strategy:
+      matrix:
+        rust: [stable, beta, nightly]
+    runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v1
     - name: Setup Rust
+      uses: hecrj/setup-rust-action@v1
+      with:
+        rust-version: ${{ matrix.rust }}
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Test
+      run: cargo test
+    - name: Coverage
+      if: matrix.rust == 'stable'
       run: |
-        rustup toolchain install nightly --profile default
-        rustup toolchain install stable
-        rustup override set stable
-    # Clippy must be run first, as its lints are only triggered during
-    # compilation. Put another way: after a successful `cargo build`, `cargo
-    # clippy` is guaranteed to produce no results. This bug is known upstream:
-    # https://github.com/rust-lang/rust-clippy/issues/2604.
-#    - name: Clippy
-#      run: cargo clippy -- --all-targets --all-features -- -D warnings
-    - name: Check formatting
-      run: |
-        cargo +nightly fmt -- --check --config-path <(echo 'license_template_path = "HEADER"')
-    - name: Build
-      run: cargo build --verbose
-    - name: Run tests
-      run: cargo test --verbose
-    - name: Run tests for all features
-      run: cargo test --verbose -- all-features
+        # Tarpaulin interoperates with CI services, however Actions is not
+        # currently supported. As a workaround, we can pretend to be Travis.
+        export TRAVIS_JOB_ID=${GITHUB_SHA}
+        export TRAVIS_PULL_REQUEST=false
+        export TRAVIS_BRANCH=${GITHUB_REF##*/}
+        cargo install cargo-tarpaulin
+        cargo tarpaulin --ciserver travis-ci --coveralls $TRAVIS_JOB_ID