This commit removes the `Reactor::run` method which has previously been used to
execute futures and turn the reactor at the same time. The tests/examples made
heavy usage of this method but they have now all temporarily moved to `wait()`
until the futures dependency is upgraded. In the meantime this'll allow us to
further trim down the `Reactor` APIs to their final state.
Note that this brings in a lot of machinery just for the sake of
wrapping file descriptors as PollEvented. This should maybe be moved
into the public API of some crate.