0.3.0-alpha.17

• Removed try_ready! macro in favor of ready!(..)?. (#1602)
• Removed io::Window::{set_start, set_end} in favor of io::Window::set. (#1667)
• Re-exported pin_utils::pin_mut! macro. (#1686)
• Made all extension traits unnamed in the prelude. (#1662)
• Allowed ?Sized types in some methods and structs. (#1647)
• Added Send + Sync bounds to ArcWake trait to fix unsoundness. (#1654)
• Changed AsyncReadExt::copy_into to consume self. (#1674)
• Renamed future::empty to pending. (#1689)
• Added #[must_use] to some combinators. (#1600)
• Added AsyncWriteExt::{write, write_vectored}. (#1612)
• Added AsyncReadExt::read_vectored. (#1612)
• Added TryFutureExt::try_poll_unpin. (#1613)
• Added TryFutureExt::try_flatten_stream. (#1618)
• Added io::BufWriter. (#1608)
• Added Sender::same_receiver and UnboundedSender::same_receiver. (#1617)
• Added future::try_select. (#1622)
• Added TryFutureExt::{inspect_ok, inspect_err}. (#1630)
• Added Compat::get_ref. (#1648)
• Added io::Window::set. (#1667)
• Added AsyncWriteExt::into_sink. (#1675)
• Added AsyncBufReadExt::copy_buf_into. (#1674)
• Added stream::pending. (#1689)
• Implemented std::error::Error for SpawnError. (#1604)
• Implemented Stream for FlattenSink. (#1651)
• Implemented Sink for TryFlattenStream. (#1651)
• Implemented AsyncRead, AsyncWrite, AsyncSeek, AsyncBufRead, FusedFuture and FusedStream for Either. (#1695)
• Replaced empty enums with Never type, an alias for core::convert::Infallible.
• Removed the futures-channel dependency from futures-sink and make futures-sink
  an optional dependency of futures-channel.
• Renamed Sink::SinkError to Sink::Error.
• Made a number of dependencies of futures-util optional.