Declare tentative return types for ext/spl - part 1 (#7115)

Fix #69264: __debugInfo() ignored while extending SPL classes

We actually implement `::__debugInfo()` and drop the `get_debug_info()`
handlers of all relevant SPL classes. This is clean

Fix #69264: __debugInfo() ignored while extending SPL classes

We actually implement `::__debugInfo()` and drop the `get_debug_info()`
handlers of all relevant SPL classes. This is cleaner and gives more
flexibility regarding overriding the functionality in descendant
classes.

show more ...