New atom-GC implementation

Hi,

Keri Harris has redesigned SWI-Prolog's atom-garbage collector. The new
code is now in the pl-devel.git repo. The old atom-gc used the
stop-the-world model, but this proved to be impossible to implement
reliably in Windows on multi-core hardware. The new atom-gc does not
stop other threads. This is a big advantage in itself. The price is that
it is now a _conservative_ garbage collector, which means it is not
guaranteed to collect all garbage atoms.

The new approach already reduces the impact of atom-gc and provides
opportunities for further improvements, notably for applications that
aim at real-time behavior.

Thanks, Keri!

	Cheers --- Jan