Add per-package changelogs

[nicholasbishop]

I've found that having a single changelog for four different packages can get a bit unwieldy. ("Do I have this entry under the right package? Is my ### header at the right level? Did I resolve these merge conflicts correctly?") Split up the big changelog into one for each package, and change the top-level one to link to the others.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[phip1611]

Love it, greate idea!