Don't write files whose contents haven't changed

[virtuald]

Let distutils try to save us some time?

It's tricky because we try to delete all the generated files, so we'd need to keep track of this, and track hashes, etc... but it would probably result in a more pleasant development experience assuming distutils does the right thing and doesn't rebuild things that aren't modified.

[virtuald]

Note that ccache and RPYBUILD_GEN_FILTER help a bit here (https://robotpy-build.readthedocs.io/en/latest/tips.html), but it would be good to get this done at some point.