memory.ml, conventions

Few remarks about memory.ml:

• use labels for the create function: let create ~(heapsize : int) ~(stacksize : int) : t =
• print_heap should be placed in heap.ml and be called print
• avoid the pattern let _ = some_expr in when the return type is unit and write some_expr; instead
• Remove the useless module path Utils. since the utils module is opened