0.3.0-alpha.19

• Stabilized the async-await feature (#1816)
• Made async-await feature no longer require std feature (#1815)
• Updated proc-macro2, syn, and quote to 1.0 (#1798)
• Exposed unstable BiLock (#1827)
• Renamed "nightly" feature to "unstable" (#1823)
• Moved to our own io::{Empty, Repeat, Sink} (#1829)
• Made AsyncRead::initializer API unstable (#1845)
• Moved the Never type from futures-core to futures-util (#1836)
• Fixed use-after-free on panic in ArcWake::wake_by_ref (#1797)
• Added AsyncReadExt::chain (#1810)
• Added Stream::size_hint (#1853)
• Added some missing FusedFuture (#1868) and FusedStream implementations (#1831)
• Added a From impl for Mutex (#1839)
• Added Mutex::{get_mut, into_inner} (#1839)
• Re-exported TryConcat and TryFilter (#1814)
• Lifted Unpin bound and implemented AsyncBufRead for io::Take (#1821)
• Lifted Unpin bounds on get_pin_mut (#1820)
• Changed SendAll to flush the Sink when the source Stream is pending (#1877)
• Set default threadpool size to one if num_cpus::get() returns zero (#1835)
• Removed dependency on rand by using our own PRNG (#1837)
• Removed futures-core dependency from futures-sink (#1832)