Extract Statistex from benchee

[PragTob]

This is step one where Statistex still lives in benchee. Goal is to have it in lib so that it could just be copy and pasted over into its own repository.

[PragTob]

I still gotta think about what the right behaviour for a lot of these functions given an empty list is:

• return nil?
• raise an ArgumentError? (Enum.min/max have this behaviour)
• return {:error, :empty_list} - currently doing this

I think one can make arguments for all of these. An argument against nil would be that for mode it is then indistinguishable from "this has no mode".

I kinda like the error tuple because it can also be put into type specs nicely.

[PragTob]

And of course now dialyzer goes wild :'( I'll look at these another time, not sure what I changed to make it go :bomb: