Feature esum psum esum only

[affeldt-aist]

Motivation for this change

closes #1954

This PR intends to extract the reusable part of PR #1954

• new lemmas about esum
• removal of fpos and fneg (superseded by funrpos and funrneg)

TODO:

• some scripts still need to be linted
• some lemmas are wrongly names
• some lemmas are not in the right files

fyi: @lyonel2017 @strub

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.

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