Replace crossbeam channel with std::sync::mpsc (#861)

* Replace crossbeam channel with std::sync::mpsc

- Replace all uses of crossbeam::channel with std::sync::mpsc
- Remove crossbeam dependency from all Cargo.toml files
- Update documentation to remove crossbeam references
- Use std::sync::mpsc::channel() instead of crossbeam::channel::unbounded()

Fixes #828

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Co-Authored-By: Claude <noreply@anthropic.com>

* Update mod.rs

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/lib.rs

@@ -25,7 +25,6 @@ pub extern crate parry3d as parry;
 #[cfg(all(feature = "dim3", feature = "f64"))]
 pub extern crate parry3d_f64 as parry;
 
-pub extern crate crossbeam;
 pub extern crate nalgebra as na;
 #[cfg(feature = "serde-serialize")]
 #[macro_use]