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_de_with.rs

@@ -0,0 +1,20 @@
+#![deny(deprecated)]
+
+use serde::Deserializer;
+use serde_derive::Deserialize;
+
+#[derive(Deserialize)]
+pub struct Struct {
+    #[serde(deserialize_with = "deprecated_with")]
+    pub field: i32,
+}
+
+#[deprecated]
+fn deprecated_with<'de, D>(_deserializer: D) -> Result<i32, D::Error>
+where
+    D: Deserializer<'de>,
+{
+    unimplemented!()
+}
+
+fn main() {}