loops4.patch adds a mechanism to avoid blocking signal catching in empty
loops (such as "for x in it: pass" or "while x: pass"). Much of the
speedup is still retained.

./python -m timeit "for x in xrange(10000): pass"
Before: 1000 loops, best of 3: 737 usec per loop
After: 1000 loops, best of 3: 438 usec per loop

./python -m timeit "x=100" "while x: x -= 1"
Before: 10000 loops, best of 3: 21.7 usec per loop
After: 100000 loops, best of 3: 16.6 usec per loop

./python Tools/pybench/pybench.py -t ForLoops
Before: 364ms per round
After: 242ms per round