Rework github actions, add code coverage (#186)

This reworks our GitHub Actions workflow to include code coverage via
tarpaulin.

Fixes #164.

.github/workflows/rust.yml

@@ -1,30 +1,65 @@
 name: Rust
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
-  build:
-    runs-on: ubuntu-latest
 
+  codestyle:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up Rust
+        uses: hecrj/setup-rust-action@v1
+        with:
+          components: rustfmt
+          # Note that `nightly` is required for `license_template_path`, as
+          # it's an unstable feature.
+          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 --all-features -- -D warnings
+
+  compile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up Rust
+        uses: hecrj/setup-rust-action@v1
+      - uses: actions/checkout@master
+      - run: cargo check --all-targets --all-features
+
+  test:
+    strategy:
+      matrix:
+        rust: [stable, beta, nightly]
+    runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v1
     - name: Setup Rust
-      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
+      uses: hecrj/setup-rust-action@v1
+      with:
+        rust-version: ${{ matrix.rust }}
+    - name: Install Tarpaulin
+      uses: actions-rs/install@v0.1
+      with:
+        crate: cargo-tarpaulin
+        version: 0.13.3
+        use-tool-cache: true
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Test
+      run: cargo test --all-features
+    - name: Coverage
+      if: matrix.rust == 'stable'
+      run: cargo tarpaulin -o Lcov --output-dir ./coverage
+    - name: Coveralls
+      if: matrix.rust == 'stable'
+      uses: coverallsapp/github-action@master
+      with:
+        github-token: ${{ secrets.GITHUB_TOKEN }}