Add collection types into the language

[denismerigoux]

Once #36 is solved, we can add a primitive polymorphic collection type into the language with its associated operators. This type should be present all the way from the surface syntax to the default calculus.

Once this type is added, the open question of #17 should be reactivated; maybe with a special map operator that maps a scope (viewed as a function) onto a collection.

Collections are fixed-length arrays, whose length can be provided at runtime. Their natural extraction target are C arrays.

[denismerigoux]

Fixed by #45