Update changelog for 0.21

Looks like I should have pulled before tagging a release

Update changelog

Use lexical-binding in tests and fix crash exposed

argdesc in byte-code function objects may be an integer rather than a
list, so handle that case gracefully.

Choose the default symbol more intelligently

Fixes #163
Closes #245

Particular thanks to @matzebond for the first implementation of this
feature!

Add bookmark support

Fixes #203

Always autoload callables if they aren't loaded

Fixes #117

Pretty-print primitive values

As of 1d2954b we pretty-print keymaps and hooks by default, but this
broke display of variables of other types. Fix that.

Add a note in the changelog for 0de7c15

Rename file to avoid confusion with other projects' unit tests

Add n and p as movement keybindings

We might want to use TAB in future for other things, perhaps
expanding/collapsing headings.