added more attribute conversions

src/mania/attributes.rs

@@ -79,4 +79,15 @@ impl ManiaPerformanceAttributes {
     pub const fn is_convert(&self) -> bool {
         self.difficulty.is_convert
     }
+
+    /// Returns a builder for performance calculation.
+    pub const fn performance<'a>(self) -> ManiaPerformance<'a> {
+        ManiaPerformance::from_mania_attributes(self.difficulty)
+    }
+}
+
+impl From<ManiaPerformanceAttributes> for ManiaDifficultyAttributes {
+    fn from(attributes: ManiaPerformanceAttributes) -> Self {
+        attributes.difficulty
+    }
 }