GroupBy.swift

extension Sequence {
    func groupBy<K: Hashable>(_ kp: KeyPath<Iterator.Element, K>) -> [K: [Iterator.Element]] {
        // Creating the resulting dictionary
        return reduce([:]) { grouped, item in
            let key = item[keyPath: kp]

            var copy = grouped
            if copy[key] == nil {
                copy[key] = []
            }
            copy[key]?.append(item)
            
            return copy
        }
    }
}