helper like ruby's Enumerable#each_slice

EachSlice.swift

extension Array {
    func eachSlice(_ step: Int) -> [[Element]] {
        return stride(from: 0, through: count, by: step)
            .lazy
            .map { from in
                let to = Swift.min(from + step, count)
                return (from..<to)
                    .lazy
                    .map { [self[$0]] }
                    .reduce([], +)
            }
    }
}