Re: gamma() and lgamma() functions

Attached is a patch adding support for the gamma and log-gamma
functions. See, for example:

https://en.wikipedia.org/wiki/Gamma_function

I think these are very useful general-purpose mathematical functions.
They're part of C99, and they're commonly included in other
mathematical libraries, such as the python math module, so I think
it's useful to make them available from SQL.

The error-handling for these functions seems to be a little trickier
than most, so that might need further discussion.

Regards,
Dean