slightly generalize fct_prodR and some other functions in functions.v

[t6s]

Motivation for this change

functions.(fct_prodE, prodrfctE, mulrfctE, exprfctE) has been stated with one of its type parameters in pointedType.
This PR simply removes this unnecessary restriction.

Checklist

- [ ] added corresponding entries in CHANGELOG_UNRELEASED.md

- [ ] added corresponding documentation in the headers

Reference: How to document

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[affeldt-aist]

Excellent!