Wrap post and pre command hooks with frames.

[lastquestion]

Post and pre command hooks are run from keyboard.c outside the command loop, so they don't have any command frame parents.

Advise add-hook so that we advise hooks added to pre and post command hooks so that the hooks themselves add frames before running the original hook.

When enabling, go through all existing hooks and make sure they are also advised.

Fixes #54.

• Needs tests
• Needs update of CHANGELOG

[lastquestion]

There's a challenge with lambda forms that can't be advised. If we change the form, then eq won't be true anymore, which means remove-hook needs to be advised for that to work. But people who directly read the values of lists would be in trouble.

I guess this is already true with timers, though; so I'll go ahead and do it anyway. No one actually reads the list of items in pre and post command hooks in all of emacs core elisp. Actually, no one even installs a lambda as a command hook, but I'm sure somewhere out there someone is doing it.