Use markers for evaluating to comments

[yuhan0]

Fixes a slightly annoying bug with comments appearing in unintended locations
when calling (expensive) async cider-eval-***-to-comment commands on a form,
then making subsequent edits while the evaluation is pending.

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by checking the relevant checkboxes):

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (make test)
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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.

[bbatsov]

The change seems reasonable to me. Please, document it in the changelog.

[yuhan0]

Alright, just edited the changelog.
I skipped the other 2 checkboxes because it's not a user-facing change to include in the manual, and would be quite difficult to write tests for this case.

[bbatsov]

Thanks!