Update crossbeam requirement from 0.7 to 0.8

[dependabot-preview[bot]]

Updates the requirements on crossbeam to permit the latest version.

Changelog

Sourced from crossbeam's changelog.

Version 0.8.0

• Bump the minimum supported Rust version to 1.36.
• Bump crossbeam-channel to 0.5.
• Bump crossbeam-deque to 0.8.
• Bump crossbeam-epoch to 0.9.
• Bump crossbeam-queue to 0.3.
• Bump crossbeam-utils to 0.8.

Version 0.7.3

• Fix breakage with nightly feature due to rust-lang/rust#65214.
• Bump crossbeam-channel to 0.4.
• Bump crossbeam-epoch to 0.8.
• Bump crossbeam-queue to 0.2.
• Bump crossbeam-utils to 0.7.

Version 0.7.2

• Bump crossbeam-channel to 0.3.9.
• Bump crossbeam-epoch to 0.7.2.
• Bump crossbeam-utils to 0.6.6.

Version 0.7.1

• Bump crossbeam-utils to 0.6.5.

Version 0.7.0

• Remove ArcCell, MsQueue, and TreiberStack.
• Change the interface of ShardedLock to match RwLock.
• Add SegQueue::len().
• Rename SegQueue::try_pop() to SegQueue::pop().
• Change the return type of SegQueue::pop() to Result.
• Introduce ArrayQueue.
• Update dependencies.

Version 0.6.0

• Update dependencies.

Version 0.5.0

• Update crossbeam-channel to 0.3.
• Update crossbeam-utils to 0.6.
• Add AtomicCell, SharedLock, and WaitGroup.

Version 0.4.1

• Fix a double-free bug in MsQueue and SegQueue.

Commits

• d9dfc9e Merge #581
• 2a3d84b Prepare for the next release
• 5ea6e70 Merge #580
• b363b3d Fix UB in destroy_array test
• e08b21c Merge #577
• 6217abf Ignore clippy::match_like_matches_macro lint
• 9cbfae7 Update cfg-if to 1
• 2444749 Merge #574
• 619f7db Fixed a few typos
• 7cc8377 Better plot.py with Python 3 & remove hard code (#569)
• Additional commits viewable in compare view

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