Double: Add log-gamma and Lambert's W

[MostAwesomeDude]

The logarithm of the gamma function, often called "log-gamma", would allow us to move nearly all of the statistics methods from RPython to pure Monte.

For completeness, if we're going to undertake such a project, we also should add Lambert's W function, which is analogous to the natural logarithm, but for x * x.exponential() instead of x.exponential().