lnutil: make LnFeatures.supports() faster

LnFeatures.supports() is became part of the hot path of
LNPathFinder.find_path_for_payment()
in 6b43eac6fd,
which made find_path_for_payment considerably slower than before.
It used to take around 0.8 sec originally, then after linked commit went to ~9 sec,
and now this takes it down to ~1.1 sec (on my laptop).

electrum/lnrouter.py

@@ -493,7 +493,7 @@ class LNPathFinder(Logger):
                 # it's ok if we are missing the node_announcement (node_info) for this node,
                 # but if we have it, we enforce that they support var_onion_optin
                 node_features = LnFeatures(node_info.features)
-                if not node_features.supports(LnFeatures.VAR_ONION_OPT):
+                if not node_features.supports(LnFeatures.VAR_ONION_OPT):  # note: this is kind of slow. could be cached.
                     return float('inf'), 0
             route_edge = RouteEdge.from_channel_policy(
                 channel_policy=channel_policy,

electrum/lnutil.py

@@ -1250,13 +1250,13 @@ class LnFeatures(IntFlag):
         you can do:
           myfeatures.supports(LnFeatures.VAR_ONION_OPT)
         """
-        enabled_bits = list_enabled_bits(feature)
-        if len(enabled_bits) != 1:
+        if (1 << (feature.bit_length() - 1)) != feature:
             raise ValueError(f"'feature' cannot be a combination of features: {feature}")
-        flag = enabled_bits[0]
-        our_flags = set(list_enabled_bits(self))
-        return (flag in our_flags
-                or get_ln_flag_pair_of_bit(flag) in our_flags)
+        if feature.bit_length() % 2 == 0:  # feature is OPT
+            feature_other = feature >> 1
+        else:  # feature is REQ
+            feature_other = feature << 1
+        return (self & feature != 0) or (self & feature_other != 0)
 
     def get_names(self) -> Sequence[str]:
         r = []