Add GitHub CI action.

- Need to use nightly toolchain atm.

.github/workflows/main.yml

@@ -0,0 +1,28 @@
+name: CI
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@master
+    - uses: actions-rs/toolchain@v1
+      with:
+        toolchain: nightly
+        override: true
+    - name: Build
+      run: cargo build --verbose
+    - name: Run tests
+      run: cargo test --verbose
+    - name: Run tests with tokio backend
+      run: cargo test --no-default-features --features tokio