Massive speedup for calculations involving many numbers with uncertainties

A calculation like str(sum(ufloat(1, 1) for _ in xrange(100000))) is now about 5,000 faster than with the latest release (version 2.4.8.1).

More generally, the calculation time of such sums (and more generally of complex expressions involving many numbers with uncertainty) is now linear instead of quadratic, and much faster in absolute time.

This closes issue #30.