Introduce GitHub Actions

The plan is to keep Travis for a short while, until we are confident that
GitHub Actions work well enough for us. And after that we can remove Travis
entirely.

There is a bunch of duplicated things but it allows us to maximize
parallelismt to have results as soon as possible.

API documentation generation is still missing.

.github/workflows/main.yml

@@ -0,0 +1,131 @@
+name: Build
+on: [ push, pull_request ]
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        php-version:
+          - '7.2'
+          - '7.3'
+          - '7.4'
+
+    name: PHP ${{ matrix.php-version }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Setup PHP, with composer and extensions
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-version }}
+          extensions: ctype, dom, gd, iconv, fileinfo, libxml, mbstring, SimpleXML, xml, xmlreader, xmlwriter, zip, zlib
+
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+
+      - name: Cache composer dependencies
+        uses: actions/cache@v2
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}
+          restore-keys: ${{ runner.os }}-composer-
+
+      - name: Install dependencies
+        run: composer install --no-progress --prefer-dist --optimize-autoloader
+
+      - name: Test with phpunit
+        run: ./vendor/bin/phpunit
+
+  php-cs-fixer:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Setup PHP, with composer and extensions
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: 7.4
+          extensions: ctype, dom, gd, iconv, fileinfo, libxml, mbstring, SimpleXML, xml, xmlreader, xmlwriter, zip, zlib
+
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+
+      - name: Cache composer dependencies
+        uses: actions/cache@v2
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}
+          restore-keys: ${{ runner.os }}-composer-
+
+      - name: Install dependencies
+        run: composer install --no-progress --prefer-dist --optimize-autoloader
+
+      - name: Code style with PHP-CS-Fixer
+        run: ./vendor/bin/php-cs-fixer fix --diff --verbose --dry-run
+
+  phpcs:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Setup PHP, with composer and extensions
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: 7.4
+          extensions: ctype, dom, gd, iconv, fileinfo, libxml, mbstring, SimpleXML, xml, xmlreader, xmlwriter, zip, zlib
+
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+
+      - name: Cache composer dependencies
+        uses: actions/cache@v2
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}
+          restore-keys: ${{ runner.os }}-composer-
+
+      - name: Install dependencies
+        run: composer install --no-progress --prefer-dist --optimize-autoloader
+
+      - name: Code style with PHP_CodeSniffer
+        run: ./vendor/bin/phpcs
+
+  coverage:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Setup PHP, with composer and extensions
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: 7.4
+          extensions: ctype, dom, gd, iconv, fileinfo, libxml, mbstring, SimpleXML, xml, xmlreader, xmlwriter, zip, zlib
+          coverage: pcov
+
+      - name: Get composer cache directory
+        id: composer-cache
+        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+
+      - name: Cache composer dependencies
+        uses: actions/cache@v2
+        with:
+          path: ${{ steps.composer-cache.outputs.dir }}
+          key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}
+          restore-keys: ${{ runner.os }}-composer-
+
+      - name: Install dependencies
+        run: composer install --no-progress --prefer-dist --optimize-autoloader
+
+      - name: Coverage
+        run: |
+          ./vendor/bin/phpunit --coverage-clover coverage-clover.xml
+          curl -LO https://scrutinizer-ci.com/ocular.phar
+          php ocular.phar code-coverage:upload --format=php-clover coverage-clover.xml