Lemmas fiberwise_{finite,countable}_preimage
#1382
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Motivation for this change
Add lemmas stating that the preimage of a set along a function is finite / countable if every pointwise preimage is so.
NB: there is already
finite_preimagewhich is similar but for injective preimages.Checklist
CHANGELOG_UNRELEASED.mdReference: How to document
Reminder to reviewers