Added method to get the unit type

src/main/java/fr/insarennes/nperier/minichamo/language/typing/BuiltinType.java

@@ -40,6 +40,12 @@ public class BuiltinType extends Type {
         super(n, sp);
     }
 
+
+    public static BuiltinType unit() {
+        return new BuiltinType("unit");
+    }
+
+
     @Override
     public Type specify(Type[] sp) {
         int arity = getArity();