Update tokio requirement from 0.1.12 to 0.2.14

[dependabot-preview[bot]]

Updates the requirements on tokio to permit the latest version.

Release notes

Sourced from tokio's releases.

Tokio v0.2.14

This release introduces automatic cooperative task yielding. This article describes the new functionality in more detail.

Fixes

• rt: concurrency bug in scheduler (#2273).
• rt: concurrency bug with shell runtime (#2333).
• test-util: correct pause/resume of time (#2253).
• time: DelayQueue correct wakeup after insert (#2285).

Added

• io: impl RawFd, AsRawHandle for std io types (#2335).
• rt: automatic cooperative task yielding (#2160, #2343, #2349).
• sync: RwLock::into_inner (#2321).

Changed

• sync: semaphore, mutex internals rewritten to avoid allocations (#2325).

Commits

• f01136b chore: prepare tokio v0.2.14 release (#2356)
• caa7e18 rt: cap fifo scheduler slot to avoid starvation (#2349)
• 7b2438e sync: fix notified link (#2351)
• 00725f6 sync: fix possible dangling pointer in semaphore (#2340)
• 5c71268 sync: broadcast, revert "Keep lock until sender notified" (#2348)
• 8020b02 fs: add coop test (#2344)
• 11acfbb rt: add task join coop test (#2345)
• f2005a7 timer: fix loom test (#2346)
• 3fb213a timer: improve memory ordering in Inner's increment (#2107)
• 6cf1a5b time: fix DelayQueue rewriting delay on insert after Poll::Ready (#2285)
• Additional commits viewable in compare view

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[dependabot-preview[bot]]

Superseded by #110.