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 8947e54d7e384a3fbec3ff4e18f0ff21226df28a.
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.