clippy: disable return_self_not_must_use

A new Clippy version added a new warning when a function that returns
`Self` doesn't have `#[must_use]`. I feel like all the cases reported
by it were false positives. Most were functions on `CommitBuilder`,
where we take `mut self` and return `Self`. I don't think I've ever
forgotten to use the result of those.

lib/src/lib.rs

@@ -15,6 +15,7 @@
 #![feature(assert_matches)]
 #![feature(map_first_last)]
 #![deny(unused_must_use)]
+#![allow(clippy::return_self_not_must_use)]
 
 #[macro_use]
 extern crate pest_derive;