Test crate with GitHub Actions

This adds a GitHub Actions workflow to test this crate with the Rust
stable, beta, and nightly toolchains.

.github/workflows/main.yml

@@ -0,0 +1,32 @@
+name: Rust
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        build: [stable, beta, nightly]
+        include:
+          - build: stable
+            rust: stable
+          - build: beta
+            rust: beta
+          - build: nightly
+            rust: nightly
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install Rust (rustup)
+        run: rustup update ${{ matrix.rust }} --no-self-update && rustup default ${{ matrix.rust }}
+        shell: bash
+      - name: Build
+        run: cargo build --verbose
+      - name: Run tests
+        run: cargo test --verbose