feat(de): Rename de::Error trait methods

serde_tests/benches/bench_struct.rs

@@ -35,7 +35,7 @@ pub enum Error {
 }
 
 impl serde::de::Error for Error {
-    fn syntax(_: &str) -> Error { Error::SyntaxError }
+    fn custom(_: String) -> Error { Error::SyntaxError }
 
     fn end_of_stream() -> Error { Error::EndOfStream }