Deadlock detection for futures::lock::Mutex

[tmandry]

It would be really nice if Mutex had deadlock detection (including via a feature flag). Especially in an async context where it's hard to peer into the state of tasks, these can be exceptionally hard to debug.

parking_lot does this for its synchronous locks, though it requires you to trigger detection manually on another thread.

cc @ctiller

[ryankurte]

v relatable problem this one, you may find no_deadlocks useful for std::sync::* objects if you have / can use them.