Consider adding contracts to withLock

[martinbonnin]

Would it be possible to add contracts for function like withLock that call their block only once?

public actual inline fun <T> ReentrantLock.withLock(block: () -> T): T {
    // Add this
    contract {
        callsInPlace(block, kind = InvocationKind.EXACTLY_ONCE)
    }
    lock()
    try {
        return block()
    } finally {
        unlock()
    }
}

This will help the compiler do better type inference and I think contracts are safe to use in libs?