Update futures requirement from 0.1 to 0.3

[dependabot-preview]

Updates the requirements on futures to permit the latest version.

Release notes

Sourced from futures's releases.

0.3.0

• Stable release along with stable async/await!
• Added async/await to default features (#1953)
• Changed Spawn trait and FuturesUnordered::push to take &self (#1950)
• Moved Spawn and FutureObj out of futures-core and into `futures-task (#1925)
• Changed case convention for feature names (#1937)
• Added executor feature (#1949)
• Moved copy_into/copy_buf_into (#1948)
• Changed SinkExt::send_all to accept a TryStream (#1946)
• Removed ThreadPool::run (#1944)
• Changed to use our own definition of io::Cursor (#1943)
• Removed BufReader::poll_seek_relative (#1938)
• Changed skip to take a usize rather than u64 (#1931)
• Removed Stream impl for VecDeque (#1930)
• Renamed Peekable::peek to poll_peek (#1928)
• Added immutable iterators for FuturesUnordered (#1922)
• Made ThreadPool optional (#1910)
• Renamed oneshot::Sender::poll_cancel to poll_canceled (#1908)
• Added some missing Clone implementations
• Documentation fixes

Changelog

Sourced from futures's changelog.

0.3.0 - 2019-11-5

• Stable release along with stable async/await!
• Added async/await to default features (#1953)
• Changed Spawn trait and FuturesUnordered::push to take &self (#1950)
• Moved Spawn and FutureObj out of futures-core and into `futures-task (#1925)
• Changed case convention for feature names (#1937)
• Added executor feature (#1949)
• Moved copy_into/copy_buf_into (#1948)
• Changed SinkExt::send_all to accept a TryStream (#1946)
• Removed ThreadPool::run (#1944)
• Changed to use our own definition of io::Cursor (#1943)
• Removed BufReader::poll_seek_relative (#1938)
• Changed skip to take a usize rather than u64 (#1931)
• Removed Stream impl for VecDeque (#1930)
• Renamed Peekable::peek to poll_peek (#1928)
• Added immutable iterators for FuturesUnordered (#1922)
• Made ThreadPool optional (#1910)
• Renamed oneshot::Sender::poll_cancel to poll_canceled (#1908)
• Added some missing Clone implementations
• Documentation fixes

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)

... (truncated)

Commits

• 0c7fa20 Stage 0.3.0 release
• 133f9ce Change stream bounds of send_all
• 4fef913 Add async/await to default features
• 47f3ccb Make Spawn::spawn take &self rather than &mut self
• 07518c5 Revert "Move AtomicWaker from futures-core to futures-task"
• 90145b2 Fix intra_doc_link_resolution_failure warnings
• 0015d2a Fix doc tests
• 5f878ee Fix no-std build
• cc31f31 Move FutureObj from futures-core to futures-task
• 6323794 Move other task utils from futures-core to futures-task
• Additional commits viewable in compare view

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