Airspeed Velocity's Stable Sort updated to Swift 3

StableSort.swift

// Swift 2 stable sort taken from AirspeedVelocity blog and updated to Swift 3
// http://airspeedvelocity.net/2016/01/10/writing-a-generic-stable-sort/

extension RangeReplaceableCollection
    where
    Index: Strideable,
    SubSequence.Iterator.Element == Iterator.Element,
IndexDistance == Index.Stride {


    public mutating func stableSortInPlace(
        isOrderedBefore: @escaping (Iterator.Element,Iterator.Element)->Bool
        ) {
        let N = self.count
        
        var aux: [Iterator.Element] = []
        aux.reserveCapacity(numericCast(N))
        
        func merge(lo: Index, mid: Index, hi: Index) {
            aux.removeAll(keepingCapacity: true)
            
            var i = lo, j = mid
            while i < mid && j < hi {
                if isOrderedBefore(self[j],self[i]) {
                    aux.append(self[j])
                    j = (j as! Int + 1) as! Self.Index
                }
                else {
                    aux.append(self[i])
                    i = (i as! Int + 1) as! Self.Index
                }
            }
            aux.append(contentsOf:self[i..<mid])
            aux.append(contentsOf:self[j..<hi])
            self.replaceSubrange(lo..<hi, with: aux)
        }
        
        var sz: IndexDistance = 1
        while sz < N {
            for lo in stride(from:startIndex, to: endIndex-sz, by: sz*2) {
                if lo.advanced(by:sz*2) >= endIndex {
                    break
                }
                merge(lo:lo, mid: lo+sz, hi:lo.advanced(by:sz*2))
            }
            sz *= 2
        }
    }
}