Use Num.bound

[pi8027]

Motivation for this change

Given R : archiNumDomainType and x : R, if we need an upper bound of x of type nat, what we need is often Num.bound (math-comp/math-comp#1510 (comment)):

Definition bound (x : R) := (truncn `|x|).+1.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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