We need this, as it will be the workhorse method to position a ImmutableBytesWritable at a given array index:

public static void positionAtArrayElement(ImmutableBytesWritable ptr, PDataType elemType, int index) {
    // Initially, ptr will point to the entire array
    // Upon return, it should point to the element at the index
    // Allocate as few objects as possible (best would be none)
}