add derives for Encode and Decode

tests/derives.rs

@@ -0,0 +1,27 @@
+#[test]
+#[cfg(feature = "macros")]
+fn encode() {
+    use sqlx::encode::Encode;
+
+    #[derive(Encode)]
+    struct Foo(i32);
+
+    #[cfg(feature = "postgres")]
+    let _: Box<dyn Encode<sqlx::Postgres>> = Box::new(Foo(1));
+    #[cfg(feature = "mysql")]
+    let _: Box<dyn Encode<sqlx::MySql>> = Box::new(Foo(1));
+}
+
+#[test]
+#[cfg(feature = "macros")]
+fn decode() {
+    use sqlx::decode::Decode;
+
+    #[derive(Decode)]
+    struct Foo(i32);
+
+    #[cfg(feature = "postgres")]
+    let _: Box<dyn Decode<sqlx::Postgres>> = Box::new(Foo(1));
+    #[cfg(feature = "mysql")]
+    let _: Box<dyn Decode<sqlx::MySql>> = Box::new(Foo(1));
+}