Add other runtimes, remove Tokio from default features

[whitfin]

There are many other popular runtimes besides Tokio, we should add modules for these runtimes too. The two in mind are smol and async_std but there are probably others.

At the same time all of these runtimes should be off by default, and opt-in via features in the dependencies. Obviously this will all be a major bump, so we can consider any changes in APIs here too.