Implement constructors type checking (?).

src/type.hpp

@@ -13,6 +13,7 @@ namespace lily {
     struct type {
         virtual ~type() = default;
         virtual bool unify_with(type* other) = 0;
+        virtual int arity() { return 0; }
     };
 
     struct type_internal : type {
@@ -49,6 +50,7 @@ namespace lily {
         type_func(type* l, type* r) :
             left(l), right(r) {}
         bool unify_with(type* other);
+        int arity() { return right->arity() + 1; }
     };
 }