Support *dictionary literal* syntax

[kazcw]

E.g. foo = { x := 1, b := 2 }. This has also been referred to as "typeset literals".

[wdanilo]

Please note that the syntax is not final and there is a discussion about what it should look like.

[JaroslavTulach]

Following syntax is supported right now, without any changes to the parser:

main = Dictionary.empty <<
    [ "x"~~10, "y"~~20, "z"~~3 ]

prints out {'y'=20, 'z'=3, 'x'=10}. All that's needed is to define these extension methods:

Dictionary.<< self add:Pair|Vector = case add of
    pair:Pair -> self.insert pair.first pair.second
    vector -> vector.fold self m-> p->
        m << p

Any.~~ self second = Pair.new self second