add zerofctE and onefctE; rename scalrfctE to scalerfctE

[t6s]

Motivation for this change

This PR adds computation lemmas for 0 and 1 in function rings:
zerofctE : 0 is the constantly zero function
onefctE : 1 is the constantly one function
They are added to the all-catch lemma fctE.

One naming fix is also included: scalrfctE -> scalerfctE.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

- [ ] added corresponding documentation in the headers

Reference: How to document

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