add trait checking

examples/strings.bl

@@ -7,3 +7,25 @@ fn main(): String {
   a = (b + c.d()); // comment
   return a.b() ;
 }
+
+type User struct {
+    id: i64
+}
+
+type TestTrait trait {
+    fn class_method(id: i64): Self;
+    fn instance_method(self: Self): i64;
+    fn default_impl(self: Self): i64;
+}
+
+impl TestTrait for User {
+    fn class_method(id: i64): Self {
+        return Self{id: id};
+    }
+    fn instance_method(self: Self): i64 {
+        return self.get_id();
+    }
+    fn default_impl(self: Self): i64 {
+        return self.instance_method();
+    }
+}