Re-export parking_lot::{Arc, Mutex} from prelude

These two types are used in public APIs, and so by re-exporting them it
makes it easier for the user to use them without needing to depend on
`parking_lot` themselves.

src/prelude.rs

@@ -16,6 +16,7 @@
 
 pub use error::Error as SerenityError;
 pub use model::Mentionable;
+pub use parking_lot::{Mutex, RwLock};
 
 #[cfg(feature = "client")]
 pub use client::{Client, ClientError as ClientError, Context, EventHandler};