add nonpoison::rwlock implementation

Adds the equivalent `nonpoison` types to the `poison::rwlock` module.
These types and implementations are gated under the `nonpoison_rwlock`
feature gate.

Also blesses the ui tests that now have a name conflicts (because these
types no longer have unique names). The full path distinguishes the
different types.

Author: connortsui20

library/std/src/sync/nonpoison.rs

@@ -33,5 +33,10 @@ impl fmt::Display for WouldBlock {
 pub use self::mutex::MappedMutexGuard;
 #[unstable(feature = "nonpoison_mutex", issue = "134645")]
 pub use self::mutex::{Mutex, MutexGuard};
+#[unstable(feature = "mapped_lock_guards", issue = "117108")]
+pub use self::rwlock::{MappedRwLockReadGuard, MappedRwLockWriteGuard};
+#[unstable(feature = "nonpoison_rwlock", issue = "134645")]
+pub use self::rwlock::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 
 mod mutex;
+mod rwlock;