It seems like most of the methods on integers are for two-argument
operations (add, mul, div, etc.), while a lot of single-argument
operations are in the math module.  If this gets added would it fit
better as a function in the math module?

I have to say a factorial function is something I've found myself
writing several times, so I certainly wouldn't mind having one included.
Yeah, it's a one-liner, but so is hypot, and that's already in the
stdlib.  And most calculators don't have a hypot button. ;)