Update tokio requirement from 0.2 to 0.3

[dependabot-preview[bot]]

Updates the requirements on tokio to permit the latest version.

Release notes

Sourced from tokio's releases.

Tokio v0.2.22

This release introduces initial support for tracing instrumentation within the Tokio runtime, enabled by the "tracing" feature flag. In addition, it contains a number of bug fixes and API additions.

Fixes

• docs: misc improvements (#2572, #2658, #2663, #2656, #2647, #2630, #2487, #2621, #2624, #2600, #2623, #2622, #2577, #2569, #2589, #2575, #2540, #2564, #2567, #2520, #2521, #2493)
• rt: allow calls to block_on inside calls to block_in_place that are themselves inside block_on (#2645)
• net: fix non-portable behavior when dropping TcpStream OwnedWriteHalf (#2597)
• io: improve stack usage by allocating large buffers on directly on the heap (#2634)
• io: fix unsound pin projection in AsyncReadExt::read_buf and AsyncWriteExt::write_buf (#2612)
• io: fix unnecessary zeroing for AsyncRead implementors (#2525)
• io: Fix BufReader not correctly forwarding poll_write_buf (#2654)
• io: fix panic in AsyncReadExt::read_line (#2541)

Changes

• coop: returning Poll::Pending no longer decrements the task budget (#2549)

Added

• io: little-endian variants of AsyncReadExt and AsyncWriteExt methods (#1915)
• task: add tracing instrumentation to spawned tasks (#2655)
• sync: allow unsized types in Mutex and RwLock (via default constructors) (#2615)
• net: add ToSocketAddrs implementation for &[SocketAddr] (#2604)
• fs: add OpenOptionsExt for OpenOptions (#2515)
• fs: add DirBuilder (#2524)

Signed-off-by: Eliza Weisman eliza@buoyant.io

Commits

• 21f7260 chore: prepare to release 0.2.22 (#2672)
• c344aac sync: support larger number of semaphore permits (#2607)
• cbb4abc chore: add audit check (#2595)
• 14723f9 doc: update links in README.md and CONTRIBUTING.md (#2609)
• 04a2826 provide a way to drop a runtime in an async context (#2646)
• 28a93e6 Update doc comments (#2572)
• dd28831 io: Forward poll_write_buf on BufReader (#2654)
• 6dcce19 sync: remove misleading comment (#2666)
• 32f46d7 time: improve Entry field comment (#2671)
• 356c81c dns: document that strings require the DNS feature (#2663)
• Additional commits viewable in compare view

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[dependabot-preview[bot]]

Superseded by #4.