Correct and improve return types for sorting functions

[johnbillion]

Various corrections and improvements to array sorting functions and methods.

• Corrects the return type of natcasesort, natsort, uasort, and uksort, these are all nullable in PHP 7
• Narrows the return types of all the array sorting functions that now return only boolean true in PHP 8
• Corrects the return types of various ArrayIterator, ArrayObject, and RecursiveArrayIterator methods which are currently marked as void but always return boolean true
• Adds all of the above to the functionMetadata files to specify that they produce side effects

The reason for narrowing bool to true is to clarify that the return value of these functions cannot be used to determine success or failure, similarly to #850.

References:

• Sort function return types: https://3v4l.org/AXUbm
• Sort method return types: https://3v4l.org/dfQkV

[ondrejmirtes]

Also please remove the merge commit (rebase cleanly on top of master).

[johnbillion]

These changes are breaking a large number of tests and I'm not familiar enough with the internals of PHPStan to know why. Closing this off, would be great if someone else could pick it up though.