Upgrade to PHPStan v0.12

Allowing us to use more advanced checks and have full generic support.

This also adds a baseline file, since we have new violations being
reported but don't want to address them right now.

composer.json

@@ -40,7 +40,7 @@
     "require-dev": {
         "doctrine/coding-standard": "^6.0",
         "jetbrains/phpstorm-stubs": "^2019.1",
-        "phpstan/phpstan": "^0.11.3",
+        "phpstan/phpstan": "^0.12",
         "phpunit/phpunit": "^8.4.1",
         "symfony/console": "^2.0.5|^3.0|^4.0|^5.0"
     },