Rename stack functions to be 'int' specific
Start using stack.h

Not all stacks are migrated (as some types are still not implemented there yet)
Make stack 'generic'

Genericness made via macros.
Additionaly move 'int_stack' testing into separate file
Move ITEM_T out of stack.h

Just before the '#include'. Type will have to be defined before the inclusion
Add general description to stack.h
Remove now redundant stack functions

Use stack.h directly.
One of the goals is to shrink (or even remove) global_variables.h
Remove guard '#define STACK_H'

Reason: this file will be inlcuded many times (given different ITME_T definitions)
Use 'stack.h' for tracking subquery openings