6.4.0 - "Factoring Fun"

Revamped factorization to be much, much faster, using the Brent-Pollard algorithm instead of just brute-force dividing. More to come...

Added the 'magnetic_constant', 'electric_constant', 'rydberg_constant', 'newtons_constant' and 'fine_structure' operators.

Added 'eulerphi' operator.

Added caching for factorizations. I often factor the same numbers over and over (like when I'm playing with the Fibonaccis) so it made sense to cache the results for non-trivial factoring.

Added the 'sigma, 'aliquot', 'polypower', 'mobius' and 'mertens' operators. The old 'mertens' operator was renamed to 'mertens_constant'. The 'aliquot' operator is another use-case for caching factorizations... try it with 276. rpn can can now factor the first 450 or so in a reasonably short time.

Added the 'frobenius', 'slice', 'sublist', 'left' and 'right' operators.

Added 'crt' operator.

...and the usual slew of bug fixes.