Stack size bugs and parisizemax

[Jeroen Demeyer <jdemeyer@cage.ugent.be>]

Hello pari-dev,

At the atelier you mentioned that it shouldn't hurt to set a small 
parisize but a large parisizemax. In the light of this, bugs where some 
computation is much slower with a small parisize are more serious.

I think there should be some general solution to this problem which 
takes into account the value of parisizemax. The idea should be: do not 
collect garbage too often if instead we can increase parisize.

For this reason, I'm not completely happy with
25b0af6b8510e5b2feaa0d47fababa2fd25921e0. My proposal would be: if 
parisize < parisizemax, then call gerepileall() at most once per outer 
loop iteration. Something like:

for (...) /* outer loop */
{
  int gc_allowed = 1;
  for (...) /* inner loop */
  {
    if (gc_allowed && gc_needed(...))
    {
      gerepileall(...);
      gc_allowed = (parisize == parisizemax);
    }
  }
}