SwapSlowFast swaps the first two entries in the memory list provided
these are both fastmemory regions. So running it on a machine without
two fastmemory regions will do nothing.

The operation is atomic (Forbid/Permit pair) and should therefore be
completely secure.

So if you've some 32bits memory followed by some 16bits fast,
You might use the following sequence;

SwapSlowFast             << Put 16bits up front
AddBuffers
SwapSlowFast             << Put 32bits back up front

To cause AddBuffers to allocate (if available) 16 bits memory.

Should also work for SlowFastMemory in the older 500/2000s. Come to
think of it, this thing is a kind of reversible "FastMemFirst".

This program is pure, and may be made resident.