make crossbeam-channels optional

[0xpr03]

Allows disabling crossbeam-channel by disabling default features.

This fixes #380

[0xpr03]

I'll leave an updated documentation to #395

[0xpr03]

One downside of this approach is that users will have to use

notify = { version = "5", default-features = false, feature=["macos_kqueue"] }

[JohnTitor]

Looks like a good approach!

One downside of this approach is that users will have to use

notify = { version = "5", default-features = false, feature=["macos_kqueue"] }

I think it's acceptable as only those who want to disable it have to touch the file, that is existing users that don't have the crossbeam issue don't have to tweak their Cargo.toml.

[nicoburns]

I wonder whether it would be worth switching wholesale to something like https://docs.rs/flume/latest/flume/ ? I believe it has very similar performance to crossbeam_channel. It might cause the same problem of course.

[0xpr03]

Unbounded MPSC seems to be the only case crossbeam-channel doesn't win over flume according to their benchmarks. Replacing our mpsc stuff with spsc would probably make more sense.

As far as I know parts of crossbeam are considered for moving into std, which would give it more credibility.

One major reason for using corssbeam-channel is probably the thread parking when waiting for channel events. Mixed up parking_lot with crossbeam-channel. CC explicitly uses more spinning to get higher performance.

[0xpr03]

If flume does prevent tokio errors, I'd be willing to accept a PR. For now my main concern is resolving the tokio scheduler issues.

This PR makes it very easy to switch it out, so if any one wants to test this, feel free. As far as I know, you can't instantly trigger this scheduler hang, so testing help is welcome for this.

Edit: Just to clairfy: Flume looks good (I've never heard of it previously), but for now I'd prefer to finish #249 once and for all. Then we can still introduce flume later on. Crossbeam seems also stuck in 0.5 limbo, but at least is has a very big userbase, so I don't fear breaking much leaving it in.