make array uniq with stored sorting

uniquified.swift

extension Array where Element: Equatable {
    var uniquified: [Element] {
        var elements = [Element]()
        forEach { if !elements.contains($0) { elements.append($0) } }
        return elements
    }
}

// because this will change sorting

var emojis = "avckg".map { String($0) }
emojis = ("avzz".map { String($0) } + emojis)
emojis = Array(Set(emojis))

// and this won't
var emojis = "avckg".map { String($0) }
emojis = ("avzz".map { String($0) } + emojis).uniquified