Numeric version of factorial()

Attached is a patch implementing factorial(), returning numeric. Points to
note:

1) arttype is numeric. I thought this was the best way of allowing
arbitarily large factorials, even though factorial(2^63) is a large
number. Happy to change to integers if this is overkill.
2) since we're accepting numeric arguments, the patch tests for floats. If
a numeric is passed with non-zero decimal portion, an error is raised
since (from memory) they are undefined.
3) I have not removed factorial([int2|int4|int8]), not their operator
counterparts since I didn't know what people would want done with these.
4) I haven't added any documentation but am happy to once I know if people
want int or numeric arguments.

Thanks,

Gavin