ext/spl: Remove spl_engine.h header

[Girgias]

This doesn't really make the calling code clearer and it is barely used on top of that.

[ndossche]

To be honest, I found the old code clearer.

[TimWolla]

Somewhat related #14254.

[Girgias]

To be honest, I found the old code clearer.

It might make sense to have a better API provided by Zend to instantiate an object and run the constructor (e.g. object_init_with_constructor(zval *arg, zend_class_entry *class_type, uint32_t param_count, zval *params)) but personally I find the names in that header rather confusing about what it is doing.

Moreover, I don't really like having other extensions using SPL headers (knowing that with the current state of iterators some extension will include them anyway)

[ndossche]

Having a better API that instantiates would be very welcome.

[Girgias]

Improved the PR after the merging of #14440