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

test_suite/tests/ui/deprecated/deprecated_ser_with.rs

@@ -0,0 +1,20 @@
+#![deny(deprecated)]
+
+use serde::Serializer;
+use serde_derive::Serialize;
+
+#[derive(Serialize)]
+pub struct Struct {
+    #[serde(serialize_with = "deprecated_with")]
+    pub field: i32,
+}
+
+#[deprecated]
+fn deprecated_with<S>(_field: &i32, _serializer: S) -> Result<S::Ok, S::Error>
+where
+    S: Serializer,
+{
+    unimplemented!()
+}
+
+fn main() {}