Adds serializer format specific field names

Allows different field names to be used for different external formats.

Field names are specified using the `rename` field attribute, e.g:

    #[serde(rename(xml= "a4", json="a5"))]

Reverts #62

Addresses #61

src/de/mod.rs

@@ -113,6 +113,10 @@ pub trait Deserializer {
     {
         self.visit(visitor)
     }
+
+    fn fmt() -> &'static str {
+        ""
+    }
 }
 
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