https://github.com/ruby/mmtk/commit/9876d8f0a1

https://github.com/ruby/mmtk/commit/de252637ec

Bumps [crossbeam-channel](https://github.com/crossbeam-rs/crossbeam) from 0.5.13 to 0.5.15.
- [Release notes](https://github.com/crossbeam-rs/crossbeam/releases)
- [Changelog](https://github.com/crossbeam-rs/crossbeam/blob/master/CHANGELOG.md)
- [Commits](https://github.com/crossbeam-rs/crossbeam/compare/crossbeam-channel-0.5.13...crossbeam-channel-0.5.15)

---
updated-dependencies:
- dependency-name: crossbeam-channel
  dependency-version: 0.5.15
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

https://github.com/ruby/mmtk/commit/f47a1e2d17

Fixes a bug where there is an infinite loop when MMTK_HEAP_MIN is small.

https://github.com/ruby/mmtk/commit/12c7ede20b

https://github.com/ruby/mmtk/commit/52b857ea04

https://github.com/mmtk/mmtk-core/pull/1261 fixes an issue where the following
script causes a Rust panic:

    GC.disable
    10_000.times { Object.new }
    puts GC.stat

https://github.com/ruby/mmtk/commit/6191ee994a

https://github.com/ruby/mmtk/commit/68bf1b638263

https://github.com/ruby/mmtk/commit/ba1ec69bf6

https://github.com/ruby/mmtk/commit/0de72c03ba

This commit implements the mark-sweep algorithm using MMTk and allows
customizing the plan using MMTK_PLAN.

https://github.com/ruby/mmtk/commit/6fea5e5ffc

Co-Authored-By: Matt Valentine-House <matt@eightbitraptor.com>