Rename named_* to use {enum,struct,unit}_*

This better reflects how they're used.

serde_tests/benches/bench_struct.rs

@@ -398,7 +398,7 @@ mod deserializer {
             }
         }
 
-        fn visit_named_map<V>(&mut self, name: &str, mut visitor: V) -> Result<V::Value, Error>
+        fn visit_struct<V>(&mut self, name: &str, mut visitor: V) -> Result<V::Value, Error>
             where V: de::Visitor,
         {
             match self.stack.pop() {