Port to GitHub Actions

Most of the Rust project has moved over to Actions, but we were still on
Travis. This sets up the needed Actions for us.

Author: steveklabnik

.github/workflows/main.yml

@@ -0,0 +1,64 @@
+name: CI
+on: [push, pull_request]
+
+jobs:
+  test:
+    name: Run tests
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: Update rustup
+      run: rustup self update
+    - name: Install Rust
+      run: |
+        rustup set profile minimal
+        rustup toolchain install 1.41.0 -c rust-docs
+        rustup default 1.41.0
+    - name: Install mdbook
+      run: |
+        mkdir bin
+        curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.3.5/mdbook-v0.3.5-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
+        echo "##[add-path]$(pwd)/bin"
+    - name: Report versions
+      run: |
+        rustup --version
+        rustc -Vv
+        mdbook --version
+    - name: Run tests
+      run: mdbook test
+  lint:
+    name: Run lints
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: Update rustup
+      run: rustup self update
+    - name: Install Rust
+      run: |
+        rustup set profile minimal
+        rustup toolchain install nightly -c rust-docs
+        rustup default nightly
+    - name: Install mdbook
+      run: |
+        mkdir bin
+        curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.3.5/mdbook-v0.3.5-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
+        echo "##[add-path]$(pwd)/bin"
+    - name: Report versions
+      run: |
+        rustup --version
+        rustc -Vv
+        mdbook --version
+    - name: Spellcheck
+      run: bash ci/spellcheck.sh list
+    - name: Lint for local file paths
+      run: | 
+        mdbook build
+        cargo run --bin lfp src
+    - name: Validate references
+      run: bash ci/validate.sh
+    - name: Check for broken links
+      run: |
+        curl -sSLo linkcheck.sh \
+          https://raw.githubusercontent.com/rust-lang/rust/master/src/tools/linkchecker/linkcheck.sh
+        # Cannot use --all here because of the generated redirect pages aren't available.
+        sh linkcheck.sh book

ci/validate.sh

@@ -0,0 +1,4 @@
+for file in src/*.md ; do
+    echo Checking references in $file
+    cargo run --quiet --bin link2print < $file > /dev/null
+done