RosettaCode factorial needs to use longs

[Jay Norwood]

After changing to longs, I made some test loops, and on release 
build dmd, pc, these are the relative times I measured for the 
different versions of factorial in that example.  So the 
iterative wins, and the functional style results in 4x penalty in 
this case.

duration factorial (hnsecs)=98
duration recFactorial (hnsecs)=151
duration fact (hnsecs)=418
duration tfactorial (hnsecs)=131

each test was like this:

sw.reset();
sw.start();
for (long i=15; i<50; i++){
   rv+=i.tfactorial;
   rv-=i.tfactorial;
}
sw.stop();
writeln("duration tfactorial (hnsecs)=", sw.peek().hnsecs);