Ignore double_ended_iterator_last clippy lint

    warning: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
       --> src/error.rs:482:22
        |
    482 |         self.chain().last().unwrap()
        |                      ^^^^^^ help: try: `next_back()`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#double_ended_iterator_last
        = note: `-W clippy::double-ended-iterator-last` implied by `-W clippy::all`
        = help: to override `-W clippy::all` add `#[allow(clippy::double_ended_iterator_last)]`

src/error.rs

@@ -478,6 +478,7 @@ impl Error {
     /// The root cause is the last error in the iterator produced by
     /// [`chain()`][Error::chain].
     #[cfg(any(feature = "std", not(anyhow_no_core_error)))]
+    #[allow(clippy::double_ended_iterator_last)]
     pub fn root_cause(&self) -> &(dyn StdError + 'static) {
         self.chain().last().unwrap()
     }