This commit removes the `Handle` argument from the following constructors
* `TcpListener::bind`
* `TcpStream::connect`
* `UdpSocket::bind`
The `Handle` argument remains on the various `*_std` constructors as they're
more low-level, but this otherwise is intended to set forth a precedent of by
default not taking `Handle` arguments and instead relying on the global
`Handle::default` return value when necesary.
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.
