Update futures-preview requirement from =0.3.0-alpha.18 to =0.3.0-alpha.19

[dependabot-preview]

Updates the requirements on futures-preview to permit the latest version.

Release notes

Sourced from futures-preview's releases.

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)

Changelog

Sourced from futures-preview's changelog.

0.3.0-alpha.19 - 2019-9-25

• 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)

0.3.0-alpha.18 - 2019-8-9

• Rewrote join! and try_join! as procedural macros to allow passing expressions (#1783)
• Banned manual implementation of TryFuture and TryStream for forward compatibility. See #1776 for more details. (#1777)
• Changed AsyncReadExt::read_to_end to return the total number of bytes read (#1721)
• Changed ArcWake::into_waker to a free function waker (#1676)
• Supported trailing commas in macros (#1733)
• Removed futures-channel dependency from futures-executor (#1735)
• Supported channel::oneshot in no_std environment (#1749)
• Added Future bounds to FusedFuture (#1779)
• Added Stream bounds to FusedStream (#1779)
• Changed StreamExt::boxed to return BoxStream (#1780)
• Added StreamExt::boxed_local (#1780)
• Added AsyncReadExt::read_to_string (#1721)
• Implemented AsyncWrite for IntoAsyncRead (#1734)
• Added get_ref, get_mut and into_inner methods to Compat01As03 and Compat01As03Sink (#1705)
• Added ThreadPool::{spawn_ok, spawn_obj_ok} (#1750)
• Added TryStreamExt::try_flatten (#1731)
• Added FutureExt::now_or_never (#1747)

0.3.0-alpha.17 - 2019-7-3

• 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)

... (truncated)

Commits

• 5d49d43 Stage 0.3.0-alpha.19
• d8e4363 Change comments at oneshot to be consistent with implementation
• 5d17474 Add FusedFuture implementations
• ad501de AtomicWaker: document memory ordering (#942)
• 4c42654 Flush sink when source stream is pending in SendAll
• ecb062f fix typo
• 160aeca Remove .rustfmt.toml
• 7400f8f Update asserted size of join! and try_join!
• ddcb889 Fixed several typos in docs
• 2d718ec Use Arc::ptr_eq in Bilock::reunite
• Additional commits viewable in compare view

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