How to group array by property in Swift

Issue #510

Use Dictionary(grouping:by:)

func groups(countries: [Country]) -> [Group] {
    let dictionary = Dictionary(grouping: countries, by: { String($0.name.prefix(1)) })
    let groups = dictionary
        .map({ (key: String, value: [Country]) -> Group in
            return Group(initial: key, countries: value)
        })
        .sorted(by: { $0.initial < $1.initial })

    return groups
}