Ignore return_self_not_must_use clippy lint

    error: missing `#[must_use]` attribute on a method returning `Self`
     --> src/display.rs:5:5
      |
    5 |     fn as_display(&self) -> Self;
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
      = note: `-D clippy::return-self-not-must-use` implied by `-D clippy::all`
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#return_self_not_must_use

src/lib.rs

@@ -205,6 +205,7 @@
     // Clippy bug: https://github.com/rust-lang/rust-clippy/issues/7421
     clippy::doc_markdown,
     clippy::module_name_repetitions,
+    clippy::return_self_not_must_use,
 )]
 
 mod aserror;