make publish might need a cleanup

[davidhewitt]

The publish job in the Makefile is a little annoying, it uses cargo hack to test the feature powerset (seems reasonable) but there are for some reason ~60 feature combinations it tests.

This took a long time to run this monrning, which made publishing a little impractical.

We should perhaps also look at ways to run release on Github so it doesn't need to run on my laptop. (This would be a nice improvement across the PyO3 org; I run too many manual publishes at the moment!)