move pop() from base class to stack class, return null if stack is empty
checking if empty is way faster than count()
move push() from base class to stack class
bottom of stack is always index zero, no need to seek
Revert "move pop() from base class to stack class, return null if stack is empty"

This reverts commit 296fcb096d9e90eabdc2adf2bf7fae77c1066534.

Conflicts:
	classes/struct/wp_sdl_struct_1_0.php
pop() should return null if list is empty
must seek to first() item because indexes keep incrementing even after an array has been emptied
wipe count cache when pop() removes an item