Replaced groups of pushes and pops with indexed access to the stack and
a single adjustment (if needed) to the stacklevel.

Avoids scores of unnecessary increments and decrements to the stackpointer.
Removes unnecessary sequential dependencies so that the compiler has more
freedom for optimizations.  Frees the processor for more parallel and
pipelined execution by using mostly read-only access and having few pointer
adjustments just prior to a read or write.