Add timing of Astropy sub-package imports

This PR adds a benchmark that times the import for each astropy sub-package.

There's a few ongoing discussions and work on changes that will affect import times, so adding a benchmark now seems like a good idea.

@mdboom Does this approach work? Are `time_this()` functions executed in fresh, independent interpreters?

If yes and this is considered useful I can add the other sub-packages.

cc @astrofrog