Qualified imports

[janmasrovira]

This pr extends the import statement to support optional renaming. The syntax is as follows:

import Stdlib.Data.Nat as Nat;
import Stdlib.Data.Nat as Qual.Nat;

It is allowed to give the same name to multiple modules:

import Stdlib.Data.Nat as X;
import Stdlib.Data.Int as X;

axiom a : X.Nat;
axiom b : X.Int;

open X; -- This will cause an ambiguous name error

[paulcadman]

Very nice - I tried it in my containers and quickcheck projects and it works well.