Modernize build automation

[x] Use GitHub Actions instead of Travis CI
[x] Have Composer in tools/composer and managed through composer self-update (see update-tools target in build.xml
[x] Install Psalm using Phive as tools/psalm (phive install --copy psalm)
[x] Install PHP-CS-Fixer using Phive as tools/php-cs-fixer (phive install --copy php-cs-fixer)
[x] Create Psalm configuration
[x] Add Psalm build step to GitHub Actions-based CI workflow
[x] Add PHP-CS-Fixer build step to GitHub Actions-based CI workflow

sebastianbergmann / recursion-context