feat(codegen): Remove {Ser,Deser}ializer::format

This feature has never been used, and it's complicating the
implementation of #207. We could restore this functionality if there is
ever interest in it.

Closes #211.

serde_codegen/src/ser.rs

@@ -612,7 +612,7 @@ fn serialize_struct_visitor<I>(
         .filter(|&(ref field, _)| !field.skip_serializing_field())
         .enumerate()
         .map(|(i, (ref field, value_expr))| {
-            let key_expr = field.serializer_key_expr(cx);
+            let key_expr = field.name_expr();
 
             let stmt = if field.skip_serializing_field_if_empty() {
                 quote_stmt!(cx, if ($value_expr).is_empty() { continue; })