Make func stat.combin.factorial public and protect against overflow

[rpoe]

What are you trying to do?

I'm working on combinatorics and permutations. I need the function factorial in my software.

What did you try?

use factorial.

How does Gonum not allow you to achieve your goal?

The function is declared private.
And it has a bug: it permits computation of factorials over 20. Range overflow without panic.

What version of Go and Gonum are you using?

v0.12.0

Is this feature absent from the current master?

no

Are you able to help contribute the feature?

I like to implement this extension.
This will be my first contribution to Gonum. so please be lenient. Thanks.

[kortschak]

What is the use case here? For a "safe" generalised implementation of factorial arbitrary width integers are needed and this is out of scope for Gonum. The factorial function here is internal for cases that are likely to have in-bound inputs, though we should probably test and document its limits.