std's `Incoming` iterator yields `TcpStream` instances. This patch
updates the `Incoming` future to match this signature.
This changes the yielded value from `(TcpStream, SocketAddr)` ->
`TcpStream`.
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 starts to add support for a global event loop by adding a
`Handle::default` method and implementing it. Currently the support is quite
rudimentary and doesn't support features such as shutdown, overriding the return
value of `Handle::default`, etc. Those will come as future commits.
Carl Lerche