Introduce `set` type

[anton-trunov]

It should support the following operations (it's a rough draft, please comment):

• emptySet(): create an empty set;
• s.exist(e): returns true if the element e is in the set and false otherwise;
• s.add(e): modify the set s by adding a new element e to it (we can discuss what this function should return, for instance it can be a boolean signaling if e was already in the set);
• s.replace(e1, e2): remove e1 and add e2 -- let's discuss the precise semantics or if this is even needed;
• s1.union(s2): s1 gets extended with the elements of s2 (we might want to also introduce a non-modifying version of this: fun union(s1: set<T>, s2: set<T>): set<T> where T is some "setable" type)
• s1.intersect(s2): s1 will keep only the elements common between s1 and s2 (the non-modifying version can be also added)
• ... (set difference, symmetric difference)
• foreach statement support for sets
• toCell(), etc.