lnutil.LnFeatures: impl and use "supports" method for feature-bit-tests

Note that for a required feature, BOLT-09 allows setting either:
- only the REQ bit
- both the REQ bit and the OPT bit

Hence, when checking if a feature is supported by e.g. an invoice, both
bits should be checked.

Note that in lnpeer.py, in self.features specifically, REQ implies OPT,
as it is set by ln_compare_features.

electrum/lnrouter.py

@@ -96,8 +96,8 @@ class RouteEdge(PathEdge):
         return True
 
     def has_feature_varonion(self) -> bool:
-        features = self.node_features
-        return bool(features & LnFeatures.VAR_ONION_REQ or features & LnFeatures.VAR_ONION_OPT)
+        features = LnFeatures(self.node_features)
+        return features.supports(LnFeatures.VAR_ONION_OPT)
 
     def is_trampoline(self):
         return False