add #[allow(deprecated)] to derive implementations

Allow deprecated in the `Serialize`/`Deserialize`
derive implementations. This allows you to
deprecate structs, enums, struct fields, or enum
variants and not get compiler warnings/errors
about use of deprecated thing. We only do this
if `#[deprecated]` or `#[allow(deprecated)]` exist
on the root object or the variants of the root
object (if it is an enum).

Resolves #2195

serde_derive/src/internals/mod.rs

@@ -1,5 +1,6 @@
 pub mod ast;
 pub mod attr;
+pub mod deprecated;
 pub mod name;
 
 mod case;