adding mutation testing, removing phpstan

.gitignore

@@ -4,3 +4,4 @@
 /.php_cs.cache
 /phpunit.clover
 /.phpunit.result.cache
+/infection.log

composer.json

@@ -36,10 +36,10 @@
     "scripts": {
 		"tests": [
 			"phpunit -vvvv",
+			"infection --min-msi=100",
 			"@static-analysis"
 		],
         "static-analysis": [
-            "phpstan analyse -c ./phpstan.neon .php_cs.dist src Tests",
             "psalm --find-dead-code --shepherd",
             "phpcpd src Tests",
             "phpmnd ./ --exclude=./symfony-http-cache/ --exclude=./phpunit/ --exclude=./vendor/ --non-zero-exit-on-violation --hint",
@@ -53,11 +53,9 @@
         "symfony/http-kernel": "^4.3"
     },
     "require-dev": {
+        "infection/infection": "^0.13.6",
         "maglnet/composer-require-checker": "^2.0",
         "php-coveralls/php-coveralls": "^2.1",
-        "phpstan/phpstan": "^0.11.15",
-        "phpstan/phpstan-phpunit": "^0.11.2",
-        "phpstan/phpstan-strict-rules": "^0.11.1",
         "phpunit/phpunit": "^8.3",
         "povils/phpmnd": "^2.1",
         "psalm/plugin-phpunit": "^0.7.0",