[Pitch] Synchronous Mutual Exclusion Lock

[Alejandro (Alejandro Alonso)]

@wadetregaskis

Opportunistic locking

I like OSAllocatedUnfairLock's withLockIfAvailable more than tryWithLock, because the "try" keyword has established meaning in Swift and it doesn't match the usage here.

I've gone ahead and updated the proposal to change the name from tryWithLock to withLockIfAvailable as I think this name reads better with the closure based API. When we get the guard APIs, I think tryLock would be the better name there, but that's a discussion for another day.