[Pitch #3] Introduce user-defined dynamically "callable" types

Evolution Pitches
54

The proposal is using DictionaryLiteral which allows duplicate keys. Its is a weird type. Further discussion about it.

func justPrinting<T>( _ input: DictionaryLiteral<String, T>)
{
    print(input)
}

justPrinting(["": 1, "": 2, "": 3]) // DictionaryLiteral<String, Int>(_elements: [("", 1), ("", 2), ("", 3) ])