Refactor explicit stacks

ssize now increases by 4 or 5 units every push.  trlink is updated
accordingly.
Convert non-function macros to constants
Replace Td with T and depth
Rename heap base pointer to HB to avoid confusion
Replace PA with an index to SA
Remove usage of ISAd and ISAn pointers
Remove usage of ISAb and ISA pointers
Convert remaining text pointers to arrays
Replace most of pointer declarations

Pointer derefecrences remain for now, but fixing them should be just a
matter of replacing by indexing into SA.