Explore GitHub Actions instead of Travis

Travis isn't building new PRs, so it seems a perfect opportunity to try
something else.

.github/workflows/check.yml

@@ -0,0 +1,68 @@
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+name: CI
+
+jobs:
+  test:
+    name: Test Suite
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust:
+          - 1.22.0
+          - stable
+          - nightly
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: ${{ matrix.rust }}
+          override: true
+      - uses: actions-rs/cargo@v1
+        with:
+          command: test
+          args: --verbose --features "strict"
+
+  fmt:
+    name: Rustfmt
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+      - run: rustup component add rustfmt
+      - uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+          args: --all -- --check
+
+  clippy:
+    name: Clippy
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+      - run: rustup component add clippy
+      - uses: actions-rs/cargo@v1
+        with:
+          command: clippy
+          args: -- -D warnings
+      - uses: actions-rs/cargo@v1
+        with:
+          command: clippy
+          # Allow deprecated because we test the no_std feature.
+          args: --all-features -- -D warnings -A deprecated