Unrolled build for #144648

Rollup merge of #144648 - connortsui20:nonpoison_rwlock, r=Mark-Simulacrum

Implementation: `#[feature(nonpoison_rwlock)]`

Tracking Issue: #134645

This PR continues the effort made in #144022 by adding the implementation of `nonpoison::rwlock`.

Many of the changes here are similar to the changes made to implement `nonpoison::mutex`. The only real difference is that this PR includes a reorganizing of the existing `poison::rwlock` file that hopefully makes both variants more readable.

### Related PRs

- `nonpoison_condvar` implementation: #144651
- `nonpoison_once` implementation: #144653

Author: rust-timer

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;