Change de::VariantVisitor to let deserializers know the variant kind

This allows formats like cbor that encode a unit variant as just a
string to work.

[breaking-change]

benches/bench_enum.rs

@@ -339,10 +339,8 @@ mod deserializer {
             de::Deserialize::deserialize(self.de)
         }
 
-        fn visit_value<V>(&mut self, visitor: V) -> Result<V::Value, Error>
-            where V: de::Visitor,
-        {
-            de::Deserializer::visit(self.de, visitor)
+        fn visit_unit(&mut self) -> Result<(), Error> {
+            de::Deserialize::deserialize(self.de)
         }
     }
 
@@ -360,7 +358,7 @@ mod deserializer {
             de::Deserialize::deserialize(self.de)
         }
 
-        fn visit_value<V>(&mut self, mut visitor: V) -> Result<V::Value, Error>
+        fn visit_seq<V>(&mut self, mut visitor: V) -> Result<V::Value, Error>
             where V: de::Visitor,
         {
             visitor.visit_seq(self)