Create method `IterableOnce#copyToArray(dest: Array[A], srcPos: Int, destPos: Int, len: Int): Int`

[joshlemer]

This would complete the functionality provided by System.arraycopy and allow for some nice speed boosts in things like:

• Iterator#copyToArray(dest, destPos, len) could be optimized in many cases by using this method, and by setting srcPosition = numElementsConsumed
• ArrayBuffer -> Vector building could be done without use of intermediate ArrayBufferIterator

This is intended for 2.14.0 milestone