Bump futures from 0.1.20 to 0.2.1

[dependabot-preview]

Bumps futures from 0.1.20 to 0.2.1.

Changelog

Sourced from futures's changelog.

0.2.1 - 2018-4-19

• Add the futures-stable crate for working with immovable futures.
• Add async/await support behind the nightly feature gate.
• Add Stream::for_each_concurrent.

0.2.0 - 2018-4-6

• Separate out the futures crate into a number of separate crates which
  are reexported by a single facade.
• Add a &mut task::Context parameter to poll methods. This type includes
  access to a wakeup callback, task-local-data, and a default executor.
• Refactor the Sink trait.

0.1.17 - 2017-10-31

• Add a close method on sink::Wait
• Undeprecate stream::iter as stream::iter_result
• Improve performance of wait-related methods
• Tweak buffered sinks with a 0 capacity to forward directly to the underlying
  sink.
• Add FromIterator implementation for FuturesOrdered and FuturesUnordered.

0.1.16 - 2017-09-15

• A prelude module has been added to glob import from and pick up a whole
  bunch of useful types
• sync::mpsc::Sender::poll_ready has been added as an API
• sync::mpsc::Sender::try_send has been added as an API

0.1.15 - 2017-08-24

• Improve performance of BiLock methods
• Implement Clone for FutureResult
• Forward Stream trait through SinkMapErr
• Add stream::futures_ordered next to futures_unordered
• Reimplement Stream::buffered on top of stream::futures_ordered (much more
  efficient at scale).
• Add a with_notify function for abstractions which previously required
  UnparkEvent.
• Add get_ref/get_mut/into_inner functions for stream take/skip methods
• Add a Clone implementation for SharedItem and SharedError
• Add a mpsc::spawn function to spawn a Stream into an Executor
• Add a reunite function for BiLock and the split stream/sink types to
  rejoin two halves and reclaim the original item.
• Add stream::poll_fn to behave similarly to future::poll_fn
• Add Sink::with_flat_map like Iterator::flat_map
• Bump the minimum Rust version to 1.13.0
• Expose AtomicTask in the public API for managing synchronization around task
  notifications.

... (truncated)

Commits

• 42332a0 Stage 0.2.1 release
• 9f8cb38 Add for_each_concurrent
• 94c9cb0 Add some async/await/StableFuture-related docs
• 0c08606 Merge pull request #972 from cramertj/either-fix
• a814d7b Add Sink::{left_sink, right_sink}
• e19e43e Deprecate and rename FutureExt::{left, right}
• b8369e2 Fix ui tests
• ae36d12 Cleanup non-Stable stuff from futures::__rt
• e0aa065 Move await back into a normal macro
• d7f61ce Fix async_block variants missing __await
• Additional commits viewable in compare view

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