Make FlagValue objects hashable

This will make them usable as set elements or dict or Counter keys.

scapy/fields.py

@@ -1039,6 +1039,8 @@ class FlagValue(object):
         self.multi = isinstance(names, list)
         self.names = names
         self.value = self._fixvalue(value)
+    def __hash__(self):
+        return hash(self.value)
     def __int__(self):
         return self.value
     def __eq__(self, other):

test/regression.uts

@@ -8185,6 +8185,8 @@ assert pkt.flags.DF
 assert pkt.flags.evil
 assert repr(pkt.flags) == '<Flag 6 (DF+evil)>'
 
+assert len({IP().flags, IP().flags}) == 1
+
 = TCP flags
 ~ TCP