Use dispatch2

[madsmtm]

The dispatch crate is unmaintained, and doesn't link on tvOS, watchOS and visionOS, building for one of those platforms currently fails with:

$ cargo +nightly build -Zbuild-std --target aarch64-apple-tvos --example readme_example
...
error: linking with `cc` failed: exit status: 1
  ...
  = note: ld: library 'dispatch' not found
          clang: error: linker command failed with exit code 1 (use -v to see invocation)

error: could not compile `ctrlc` (example "readme_example") due to 1 previous error

I maintain an alternative called dispatch2, which works on these platforms, and also has a safer API (you'd be able to fully avoid the unsafe here if you used a static OnceLock instead of static mut).

This is similar in spirit to my previous PR #128.

[Detegr]

Thanks!