Mutex Deadlock fix: Track the fiber that owns a Mutex - #9649

The Mutex deadlock check asked the owning thread's ThreadContext which fiber it was currently running. That answer is only right while the root fiber holds the lock. The reverse case is a fiber holding the lock while the root fiber tries to take it. There the check never fires, and Mutex#lock blocks forever instead of raising ThreadError. Store the owning fiber on the mutex instead.

Unexcludes TestFiberMutex#test_mutex_deadlock.

Sample code:

m = Mutex.new
f = Fiber.new { m.lock; Fiber.yield }
f.resume
m.lock

Master: hangs
This branch: ThreadError: deadlock; lock already owned by another fiber belonging to the same thread