CHANGELOG: Penultimate updates (hopefully) for 1.4.0

Ian Jacksonrequested to merge
Diziet/arti:changelog-2 into main

This contains all the changes, I believe. (cargo-semver-checks is still running.)

I have not yet done the systematic reformatting into the fiddly ([...] [...]) format. Let's review the content before we make it hard to edit.

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