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/deprecated.rs

@@ -0,0 +1,34 @@
+#![deny(deprecated)]
+#![allow(dead_code)]
+
+use serde_derive::{Deserialize, Serialize};
+
+/// deprecated enum
+#[derive(Serialize, Deserialize)]
+#[deprecated]
+enum E1 {
+    A,
+    B,
+}
+
+/// deprecated struct
+#[derive(Serialize, Deserialize)]
+#[deprecated]
+struct S1 {
+    a: bool,
+}
+
+/// deprecated enum variant
+#[derive(Serialize, Deserialize)]
+enum E2 {
+    A,
+    #[deprecated]
+    B,
+}
+
+/// deprecated struct field
+#[derive(Serialize, Deserialize)]
+struct S2 {
+    #[deprecated]
+    a: bool,
+}