Migrate CI to GitHub Actions

This commit migrates from Azure Pipelines to Github Actions for CI for
cranelift. The CI configuration was relatively straightforward, and the
intention here is not to change what's actually being done on CI, just
change where it's being done. The previous CI configuration had build
targets for producing releases, but these weren't actually applicable
for cranelift itself (mostly just copied from wasmtime), so they've been
folded into the main "test everything" matrix which now includes
`--release` mode items.

See cranestation/wasmtime#474 for some more context as well in terms of
benefits of Github Actions vs Azure Pipelines.

cranelift/.github/workflows/main.yml

@@ -0,0 +1,136 @@
+name: CI
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  rustfmt:
+    name: Rustfmt
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+      with:
+        submodules: true
+    - name: Install Rust
+      run: rustup update stable && rustup default stable && rustup component add rustfmt
+    - run: cargo fmt -- --check
+
+  docs:
+    name: Build API Docs
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+      with:
+        submodules: true
+    - name: Install Rust
+      run: rustup update nightly && rustup default nightly
+    - run: cargo doc --all --exclude cranelift-codegen-meta
+    - run: cargo doc --package cranelift-codegen-meta --document-private-items
+    - run: cargo install cargo-deadlinks
+    - run: find ./target/doc -maxdepth 1 -type d -name "cranelift*" | xargs -I{} cargo deadlinks --dir {}
+      name: Run cargo-deadlinks
+
+  test:
+    name: Test
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        # first a list of everything we test...
+        name:
+        - windows-earliest
+        - linux-earliest
+        - mac-earliest
+        - stable
+        - beta
+        - nightly
+        - windows-release
+        - linux-release
+        - mac-release
+        # .. and then the actual configuration for each thing we test
+        include:
+        - name: windows-earliest
+          os: ubuntu-latest
+          rust: 1.37.0
+        - name: linux-earliest
+          os: ubuntu-16.04
+          rust: 1.37.0
+        - name: mac-earliest
+          os: macos-10.14
+          rust: 1.37.0
+        - name: stable
+          os: macos-latest
+          rust: stable
+        - name: beta
+          os: macos-latest
+          rust: beta
+        - name: nightly
+          os: macos-latest
+          rust: nightly
+        - name: mac-release
+          os: macos-latest
+          rust: stable
+          release: --release
+        - name: linux-release
+          os: ubuntu-latest
+          rust: stable
+          release: --release
+        - name: windows-release
+          os: windows-latest
+          rust: stable
+          release: --release
+    steps:
+    - uses: actions/checkout@master
+      with:
+        submodules: true
+
+    - name: Install Rust (rustup)
+      run: rustup update ${{ matrix.rust }} --no-self-update && rustup default ${{ matrix.rust }}
+      if: "!startsWith(matrix.os, 'macos-')"
+      shell: bash
+    - name: Install Rust (macos)
+      run: |
+        curl https://sh.rustup.rs | sh -s -- -y --default-toolchain=${{ matrix.rust }}
+        echo ::add-path::$HOME/.cargo/bin
+      if: startsWith(matrix.os, 'macos-')
+
+    - run: cargo fetch
+    - run: cargo build ${{ matrix.release }}
+    - run: cargo test --all ${{ matrix.release }}
+      env:
+        RUST_BACKTRACE: 1
+
+    # Ensure fuzzer works by running it with a single input
+    - run: cargo install cargo-fuzz
+      if: matrix.rust == 'nightly'
+    - run: |
+        fuzz_module="ffaefab69523eb11935a9b420d58826c8ea65c4c"
+        cargo fuzz run fuzz_translate_module fuzz/corpus/fuzz_translate_module/$fuzz_module
+      env:
+        RUST_BACKTRACE: 1
+      if: matrix.rust == 'nightly'
+      continue-on-error: true
+
+  fuzz:
+    name: Fuzz Regression
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: Install Rust
+      run: rustup update nightly && rustup default nightly
+    - run: cargo install cargo-fuzz
+    - run: ci/fuzzit.sh local-regression
+
+  fuzz_push:
+    name: Fuzz (push)
+    runs-on: ubuntu-latest
+    if: github.event_name == 'push'
+    steps:
+    - uses: actions/checkout@master
+    - name: Install Rust
+      run: rustup update nightly && rustup default nightly
+    - run: cargo install cargo-fuzz
+    - run: ci/fuzzit.sh fuzzing
+      env:
+        FUZZIT_API_KEY: ${{ secrets.FUZZIT_API_KEY }}