Reuse temp registers to reduce instruction count

Instruction count for qsort is down 14 instructions.
Gain another 13 instructions from qsort

This time we make swap() from a macro into a macro that calls a
function, do_swap(). This means the comparison against the existing
C quicksort benchmark is a bit skewed in favour of the tenyr version,
until the C version is given the opportunity to use a function.
Make qsort comparison more fair

Use a function in qsort.c to make the comparison to the new
do_swap()-enabled code in qsort.tas.cpp more fair. Remove some
unnecessary stack traffic in qsort.tas.cpp. Although both codes are now
smaller, the size of the tenyr implementation relative to the x86-64
implementation is now greater than it was before the macro to function
conversion.
Increasing competitiveness of tenyr quicksort

Better code density through better register allocation / lifetime.
Mend segfault with prealloc ram