There have been requests to add other C99 libm functions to Python:
expm1 is requested in issue 3501, and log2 came up in the issue 3724 
discussion.  I'm absorbing those issues into this one.

With these two, and the gamma and error functions, we're pretty close to 
implementing all the functions in C99s math.h.  Seems to me that we might 
as well aim for completeness and implement the whole lot.