Add set of lemmas for esum

[lyonel2017]

Motivation for this change

Set of lemmas for esum extracted from #2049.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reminder to reviewers

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

[affeldt-aist]

Sorry for the random, minor comments.
Yet, I am already concerned by the fact that we have summable defined both in esum.v and in realsum.v: we might want to distinguish one or the other by adding a small e or a small r in the identifier.