add lemma lt0_adde

[affeldt-aist]

Motivation for this change

fixes #1714

@hoheinzollern In fact, this lemma is coming from Natalia's development.
But since it used there, it is fair to commit it to MCA.
I renamed it to lt0_adde since adde_lt0 rather suggests a goal of the shape _ + _ < 0, which is not the case.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

- [ ] added corresponding documentation in the headers

Reference: How to document

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• 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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[affeldt-aist]

ping @hoheinzollern

[hoheinzollern]

lgtm