Export types provided by arc_lock feature.

This adds conditional re-exports of the Arc-related types from the lock_api crate.

src/lib.rs

@@ -54,3 +54,6 @@ pub use self::rwlock::{
     RwLockUpgradableReadGuard, RwLockWriteGuard,
 };
 pub use ::lock_api;
+
+#[cfg(feature = "arc_lock")]
+pub use self::lock_api::{ArcMutexGuard, ArcReentrantMutexGuard, ArcRwLockReadGuard, ArcRwLockUpgradableReadGuard, ArcRwLockWriteGuard};