RawValue -> repr(transparent)

This code is from September 2018, at which point serde_json supported
rustc back to 1.15. repr(transparent) stabilized in 1.28. Today the
minimum required compiler is 1.36, so repr(transparent) is okay to use.

Author: dtolnay

src/raw.rs

@@ -112,7 +112,7 @@ use serde::ser::{Serialize, SerializeStruct, Serializer};
 ///     raw_value: Box<RawValue>,
 /// }
 /// ```
-#[repr(C)]
+#[repr(transparent)]
 #[cfg_attr(docsrs, doc(cfg(feature = "raw_value")))]
 pub struct RawValue {
     json: str,