Feature: multiple persistent eval overlays

[yuhan0]

This adds a third option 'change to cider-eval-result-duration, which persists eval overlays until there is a change to the buffer, allowing for easy comparisons between different forms.

A quick demo:

Note: This is how Calva behaves, see the demo at https://calva.io/try-first/

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Thanks!

If you're just starting out to hack on CIDER you might find this section of its
manual extremely useful.

[vemv]

Perhaps this comment could be more verbose?

e.g. NOTE: please don't use remove-overlays, as it splits and leaves behind partial overlays. See <source>

[yuhan0]

<source> referring to the function's docstring explaining this behaviour or the resulting issue?
Here's the problem that arises with multiple overlays and splitting:

[bbatsov]

I guess to the issue, but I don't have a strong preference here, as long as it's clear what we're avoiding.

[bbatsov]

I like the proposed change. Let's just document this somewhere (I'm not sure if we've even documented the old two options) and illustrated the differences with a couple of animated gifs like the one you've used in the PR description.

[yuhan0]

Ok, I've updated the changelog and manual, also another commit that I accidentally missed out from the last PR.
I also realise that my title above was inaccurate, currently setting the variable to a number or nil currently does allow for multiple overlays on screen.

[bbatsov]

Thanks!