Rewrite argument type checks as assertions

[ricardoboss]

According to https://www.php.net/manual/en/function.assert.php, the assert keyword is, since PHP 7, a language construct, which is discarded in a production runtime and executed in a development scenario. There are multiple type checks for method arguments (for example for implementations of ArrayAccess), which also help improve static analysis. The same can be achieved with assertions while improving real-world performance, since they are not evaluated if the appropriate settings are in place.

Method Elephox\Collection\ObjectSet::add already implements the desired assertion style.