We can now create set and dictionary literals.

For example:

    [:set: 1 2 3]  # creates the set {1,2,3}
    [:dict: 1 "A" 2 "B"]  # creates the dict {1:"A", 2:"B"}

There are also similar :list: and :tuple: tags, but they should never
be used (since [...] can create a list and (…) can create a tuple).
Created constants for all literals in Parser class.