replace `posnum.v` by `signed.v` from the forthcoming `coq-mathcomp-reals` package?

[affeldt-aist]

apery/theories/posnum.v

Line 1 in da803b7

(* mathcomp analysis (c) 2017 Inria and AIST. License: CeCILL-C. *)

I noticed that this repository contains a copy of posnum.v from a past version of MathComp-Analysis.
It has since been reimplemented (and improved) as signed.v and it will soon be made available as a lightweight
package (coq-mathcomp-reals? see https://github.com/math-comp/analysis/tree/master/reals) so that you can use it without needing MathComp-Analysis. @proux01

That would also contribute to solve issue #10 .