Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features.

crossbeam-channel cfg(unix) implicit feature

With default (std)

parking_lot cfg(all(unix, any(target_os = "espidf", target_os = "horizon", target_os = "openbsd", target_os = "redox", target_os = "tvos", target_os = "vxworks"))) implicit feature