Use modern syntax for optional dependencies #1229

Use the "dep:" prefix to specify optional dependencies in Cargo features. An optional crate dependency implicitly generates a feature of the same name, and can be leaked to the user of the current crate. But if a feature specifies a crate dependency with the "dep:" prefix, it will not implicitly generate the feature, hiding it from the users. The "dep:" prefix was introduced in Rust 1.60.