Fix missing import

Author: mmirate

src/lib.rs

@@ -59,6 +59,8 @@ use alloc::{
 
 pub use either::Either;
 
+#[cfg(feature = "use_std")]
+use std::borrow::Borrow;
 #[cfg(feature = "use_std")]
 use std::collections::HashMap;
 use std::iter::{IntoIterator, once};