Fix eta-expansion in evaluator (copy #2782)

[mergify]

For some eta-reduced 'e', we used to bogusly eta-expand to:

\x.(\y. e y) x

We now correctly expand to:

\x.\y.(e x) y

Fixes #2781

Still TODO:

• Write a changelog entry (see changelog/README.md)
• Check copyright notices are up to date in edited files

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This is an automatic copy of pull request #2782 done by [Mergify](https://mergify.com).