Update crossbeam requirement to 0.4

[dependabot-preview]

Updates the requirements on crossbeam to permit the latest version.

Changelog

Sourced from crossbeam's changelog.

Version 0.4

• Switch to the new implementation of epoch-based reclamation in
  crossbeam-epoch, fixing numerous bugs in the
  old implementation. Its API is changed in a backward-incompatible way.
• Switch to the new implementation of CachePadded and scoped thread in
  crossbeam-utils. The scoped thread API is
  changed in a backward-incompatible way.
• Switch to the new implementation of Chase-Lev deque in
  crossbeam-deque. Its API is changed in a
  backward-incompatible way.
• Export channel implemented in
  crossbeam-channel.
• Remove AtomicOption.
• Implement Default and From traits.

Version 0.3

• Introduced ScopedThreadBuilder with the ability to name threads and set stack size
• Worker methods in the Chase-Lev deque don't require mutable access anymore
• Fixed a bug when unblocking pop() in MsQueue
• Implemented Drop for MsQueue, SegQueue, and TreiberStack
• Implemented Default for TreiberStack
• Added is_empty to SegQueue
• Renamed mem::epoch to epoch
• Other bug fixes

Version 0.2

• Changed existing non-blocking pop methods to try_pop
• Added blocking pop support to Michael-Scott queue
• Added Chase-Lev work-stealing deque

Version 0.1

• Added epoch-based memory management
• Added Michael-Scott queue
• Added Segmented array queue

Commits

• See full diff in compare view

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[rust-highfive]

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