suchThat()

Probably a name easier to understand that implies().

* suchThat() takes a single anonymous function
* suchThat() takes independent constraints for each argument (but still evaluates the whole tuple of values)
* multiple suchThat() are possible
* if the ratio of `actually evaluated` to `generated` is under the configurable 0.5 threshold, the test fails
To stay more close to the doc-block, what about:

```php
else if (is_callable($arguments)) {
    // ...
}
```

This make us a little bit safe because `SingleCallbackAntecedent` has no type hinting and there we want a callback. If it is not a callback then we throw the exception to signal the "Invalid call to suchThat".