Add `Int*`, `Word*`, `Identity` and `Maybe` instances for `Counter`

[martijnbastiaan]

Fixes #2534

Still TODO:

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

[martijnbastiaan]

@gergoerdi Could you check whether this does what you want?

[gergoerdi]

This doesn't seem to address exporting counterMin.

[martijnbastiaan]

This doesn't seem to address exporting counterMin.

You're right, I've exported the internal functions now. We haven't thought of anything that would need changing.