factorial

[cirocosta]

Prepare a split-recursive approach or something better for calculating the factorials.

• http://www.cs.berkeley.edu/~fateman/papers/factorial.pdf
• http://en.literateprograms.org/Factorials_with_prime_factorization_%28Python%29
• http://www.luschny.de/math/factorial/FastFactorialFunctions.htm
• http://bugs.python.org/issue8692

[cirocosta]

Note: we MUST NOT rely on approximations. This must be exactly (i.e, no shirling's approach here)