Update tokio requirement from 0.2 to 1.4

[dependabot-preview[bot]]

Updates the requirements on tokio to permit the latest version.

Release notes

Sourced from tokio's releases.

Tokio v1.4.0

Added

• macros: introduce biased argument for select! (#3603)
• runtime: add Handle::block_on (#3569)

Fixed

• runtime: avoid unnecessary polling of block_on future (#3582)
• runtime: fix memory leak/growth when creating many runtimes (#3564)
• runtime: mark EnterGuard with must_use (#3609)

Documented

• chore: mention fix for building docs in contributing guide (#3618)
• doc: add link to PollSender (#3613)
• doc: alias sleep to delay (#3604)
• sync: improve Mutex FIFO explanation (#3615)
• timer: fix double newline in module docs (#3617)

#3564: tokio-rs/tokio#3564 #3613: tokio-rs/tokio#3613 #3618: tokio-rs/tokio#3618 #3617: tokio-rs/tokio#3617 #3582: tokio-rs/tokio#3582 #3615: tokio-rs/tokio#3615 #3603: tokio-rs/tokio#3603 #3609: tokio-rs/tokio#3609 #3604: tokio-rs/tokio#3604 #3569: tokio-rs/tokio#3569

Commits

• dcac336 chore: prepare Tokio v1.4.0 (#3621)
• c39d986 runtime: add Handle::block_on (#3569)
• e4f7668 runtime: fix memory leak/growth when creating many runtimes (#3564)
• e6103d6 docs: add link to PollSender (#3613)
• cc90a5c chore: mention fix for building docs in contributing guide (#3618)
• f107c4f timer: fix double newline in module docs (#3617)
• e6a9167 runtime: avoid unnecessary polling of block_on future (#3582)
• 345b29c sync: improve Mutex FIFO explanation (#3615)
• b75d02a macros: introduce biased argument for select! (#3603)
• a1b4bde runtime: mark EnterGuard with must_use (#3609)
• Additional commits viewable in compare view

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[dependabot-preview[bot]]

Superseded by #29.