Vec2: re-enable the arg method

crates/cross/src/vec.rs

@@ -138,11 +138,14 @@ impl<R: Real> Vec2<R> {
         }
     }
 
-    // /// Returns the angle of this point, (-pi, pi]
-    // pub fn arg(&self) -> R {
-    //     // self.y.atan2(self.x)
-    //     self.y.to_f64().atan2(self.x.to_f64()).cast()
-    // }
+    /// returns the angle of this point, (-pi, pi]
+    /// requires std feature
+    #[cfg(feature = "std")]
+    pub fn arg(&self) -> R {
+        // self.y.atan2(self.x)
+        // TODO: add `Real::atan2` and remove these casts
+        self.y.to_f64().atan2(self.x.to_f64()).cast()
+    }
 
     pub fn rotate(&self, angle: R) -> Self {
         let (sin, cos) = angle.sin_cos();