Add `temper clean` subcommand

[OliveIsAWord]

It would be nice to have a subcommand to delete build artifacts, e.g. the temper.keep and temper.out directories.

[tjpalmer]

I agree some form of temper clean could be convenient, so we can look into that.

Note, however, that temper.keep ideally is meant to stored in source control. It's not put to proper use yet, though, and we have some open questions on how much should be stored there. See #283 on that some of that. But the intent is to use this information to ensure api stability in libraries over time. Currently that's just name choice information, but we want to track all relevant information in the future as well.

So temper clean might in the end be just rm -r temper.out, but sometimes it's nicer to work at a higher semantic level instead of manual destructive file system actions.

[mikesamuel]

temper.keep needs to become something that distinguishes between different kinds of info:

• info pertaining to a published version of a library, e.g. something uploaded to pypi which we now need to remain backwards compatible with
• info that is relevant to the current, local dev build, i.e. choices that are useful to remember but which are not fixed in stone because there's no artifact out there that entails a compatibility requirement

temper clean could throw away the latter.

@OliveIsAWord Who/how do you envision using temper clean?

Is this something that you'd want CI/CD scripts do before making a build with ENV=production after having made a previous build with ENV=test or somesuch?

After checking out a different branch?

An equivalent of turn off and turn back on after seeing something weird?