Make tor-rtcompat compileable without any runtimes

Right now tor-rtcompat can't be built with no-default-features unless something else adds the desired runtime back in. This made it impossible to publish the crates that used it; I had to kludge the issue with 8947e54d.

Before 0.0.1, we should fix this whole business somehow?

I'd like it to be so that every module's tests can run without any special options, but where we don't need to no-default-features in all the intermediare modules.