Support deserialization of struct keys from integers

serde-cbor supports a "packed" serialization flag which causes keys to
be serialized as their indices, but the deserializer currently has to
hardcode support for this format. We can simply support deserialization
of struct keys from integers as we already do for enum variants.

test_suite/tests/test_de.rs

@@ -605,6 +605,17 @@ declare_tests! {
             Token::StructEnd,
         ],
     }
+    test_struct_integer_keys {
+        Struct { a: 1, b: 2, c: 0 } => &[
+            Token::Struct { name: "Struct", len: 2 },
+                Token::U32(0),
+                Token::I32(1),
+
+                Token::U32(1),
+                Token::I32(2),
+            Token::StructEnd,
+        ],
+    }
     test_enum_unit {
         Enum::Unit => &[
             Token::UnitVariant { name: "Enum", variant: "Unit" },