Make Vector::uncheckedAppend() an alias to Vector::append() so that bounds checking happens. This is part of our effort to make our code safer. This tested as performance neutral on Speedometer, MotionMark and JetStream on various A/B bots. However, I plan to monitor the bots after landing to be safe. If it sticks, I'll follow-up to get rid of Vector::uncheckedAppend() completely. Note that in order to maintain the performance, WTF::map(), Vector::map(), copyToVector() & copyToVectorOf() still rely on a private unsafeAppendWithoutCapacityCheck() function.