Move last piece of ZArray to GrowableArray

ZArray used to be a separate implementation of a dynamically allocated/growable array. It now instead inherits from GrowableCHeapArray, and extends it with a transfer() function. I propose we rename this function to swap() and move it to GrowableArrayWithAllocator, since this function could be generally useful. It would also mean that ZArray could be just a typedef/using of GrowableCHeapArray.