Adapt to rocq#19987

[Tragicus]

Motivation for this change

rocq-prover/rocq#19987 refolds terms before using them to instantiate evars. In classical/functions.v, there is a lemma (image_subP) that uses homomorphism_1, which introduces eta-expansions. Due to unfoldings in the previous algorithm, these eta-expansions were removed, whereas they are not now, which makes rewrite's matching algorithm fail. In theories/function_spaces, there is a second order unification problem which is solved differently.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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