Force garbage collection.
This function checks all the memory for cyclic structures such
as arrays containing themselves and frees them if appropriate.
It also frees up destructed objects and things with only weak
references. It then returns how many
arrays/objects/programs/etc. it managed to free by doing this.
Normally there is no need to call this function since Pike will
call it by itself every now and then. (Pike will try to predict
when 20% of all arrays/object/programs in memory is 'garbage'
and call this routine then.)