std: Return Result from RWLock/Mutex methods
All of the current std::sync primitives have poisoning enable which means that when a task fails inside of a write-access lock then all future attempts to acquire the lock will fail. This strategy ensures that stale data whose invariants are possibly not upheld are never viewed by other tasks to help propagate unexpected panics (bugs in a program) among tasks. Currently there is no way to test whether a mutex or rwlock is poisoned. One method would be to duplicate all the methods with a sister foo_catch function, for example. This pattern is, however, against our [error guidelines][errors]. As a result, this commit exposes the fact that a task has failed internally through the return value of a `Result`. [errors]: https://github.com/rust-lang/rfcs/blob/master/text/0236-error-conventions.md#do-not-provide-both-result-and-fail-variants All methods now return a `LockResult<T>` or a `TryLockResult<T>` which communicates whether the lock was poisoned or not. In a `LockResult`, both the `Ok` and `Err` variants contains the `MutexGuard<T>` that is being returned in order to allow access to the data if poisoning is not desired. This also means that the lock is *always* held upon returning from `.lock()`. A new type, `PoisonError`, was added with one method `into_guard` which can consume the assertion that a lock is poisoned to gain access to the underlying data. This is a breaking change because the signatures of these methods have changed, often incompatible ways. One major difference is that the `wait` methods on a condition variable now consume the guard and return it in as a `LockResult` to indicate whether the lock was poisoned while waiting. Most code can be updated by calling `.unwrap()` on the return value of `.lock()`. [breaking-change]
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