Mertens theorems

[Tragicus]

Motivation for this change

Proof of Mertens theorems.

This still requires a bit of prettyfiyng, but the proof is complete if someone wants to take a look. Dependecies:

• https://github.com/math-comp/analysis/pull/1705

Checklist

• [ ] added corresponding entries in CHANGELOG_UNRELEASED.md

• [ ] added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• PRs with several commits that make sense individually and that all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
• Check labels