Ignore return_self_not_must_use clippy lint

    error: missing `#[must_use]` attribute on a method returning `Self`
       --> src/lib.rs:437:5
        |
    437 | /     pub fn resolved_at(&self, other: Span) -> Span {
    438 | |         Span::_new(self.inner.resolved_at(other.inner))
    439 | |     }
        | |_____^
        |
        = 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

    error: missing `#[must_use]` attribute on a method returning `Self`
       --> src/lib.rs:443:5
        |
    443 | /     pub fn located_at(&self, other: Span) -> Span {
    444 | |         Span::_new(self.inner.located_at(other.inner))
    445 | |     }
        | |_____^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#return_self_not_must_use

src/lib.rs

@@ -98,6 +98,7 @@
     clippy::manual_assert,
     clippy::must_use_candidate,
     clippy::needless_doctest_main,
+    clippy::return_self_not_must_use,
     clippy::shadow_unrelated,
     clippy::trivially_copy_pass_by_ref,
     clippy::unnecessary_wraps,