Prove some admitted lemmas in experimental_reals

[ethanlee515]

Motivation for this change

Prove three admitted lemmas in experimental_reals/realsum.v:

• psumB
• interchange_sup
• interchange_psum

Un-deprecate the following, as they no longer depend on unproven lemmas:

• dlet_dlet
• dmargin_dlet
• dlet_dmargin
• dfst_dswap
• dsnd_dswap
• dsndE
• pr_dlet

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.

Reminder to reviewers

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[affeldt-aist]

Excellent! Note that psumB was also proved in the meantime in https://github.com/math-comp/analysis/pull/1978 (under review). fyi @t6s @strub @lyonel2017