feat: add multiple channel providers

Each provider is enabled via a feature flag.
The currently implemented providers are:
- std::mpsc
- flume
- crossbeam_channel

src/sync/mod.rs

@@ -0,0 +1,17 @@
+pub mod traits;
+
+#[cfg(feature = "sync-crossbeam")]
+pub mod crossbeam;
+#[cfg(feature = "sync-flume")]
+pub mod flume;
+#[cfg(feature = "sync-std")]
+pub mod std;
+
+#[cfg(feature = "sync-flume")]
+pub use flume::FlumeSuck;
+
+#[cfg(feature = "sync-crossbeam")]
+pub use crossbeam::CrossbeamSuck;
+
+#[cfg(feature = "sync-std")]
+pub use std::StdSuck;