Function getSortedIndex

Given a sorted array, a value, and a sorting function, returns the index where the value should be inserted.

Notes:

• If the input array is not sorted, it won't work. There is no validation to make sure that the array you put in is sorted, either.
• The algorithm works in a similar way to quicksort. It pivots on the centre of the array, and figures out which side the value will fall on, then pivots on the centre of that side, then again and again until the correct location is found. This has the effect of heavily reducing the number of required sorting function calls.