PHPStan extension installer, baseline, fixes (#602)

* Use PHPStan extension installer

* Add few return types to anonymous functions

* Add PHPStan baseline

Author: simPod

composer.json

@@ -16,6 +16,7 @@
   "require-dev": {
     "doctrine/coding-standard": "^6.0",
     "phpbench/phpbench": "^0.14",
+    "phpstan/extension-installer": "^1.0",
     "phpstan/phpstan": "0.12.2",
     "phpstan/phpstan-phpunit": "0.12.1",
     "phpstan/phpstan-strict-rules": "0.12.0",
@@ -52,6 +53,7 @@
     "lint" : "phpcs",
     "fix" : "phpcbf",
     "stan": "phpstan analyse --ansi --memory-limit 2048M",
+    "phpstan-baseline": "phpstan analyse --ansi --error-format baselineNeon > phpstan-baseline.neon",
     "check": "composer lint && composer stan && composer test"
   }
 }