Implement Borrow for NotNan

This is limited to `NotNan<f32>` and `NotNan<f64>` because it would be
incorrect for custom numeric types that have their own `Hash`
implementations that do not match the one for `NotNan`.

Fixes #98

src/lib.rs

@@ -9,6 +9,7 @@ extern crate std;
 #[cfg(feature = "std")]
 use std::error::Error;
 
+use core::borrow::Borrow;
 use core::cmp::Ordering;
 use core::convert::TryFrom;
 use core::fmt;
@@ -915,6 +916,20 @@ impl<T: Float> AsRef<T> for NotNan<T> {
     }
 }
 
+impl Borrow<f32> for NotNan<f32> {
+    #[inline]
+    fn borrow(&self) -> &f32 {
+        &self.0
+    }
+}
+
+impl Borrow<f64> for NotNan<f64> {
+    #[inline]
+    fn borrow(&self) -> &f64 {
+        &self.0
+    }
+}
+
 #[allow(clippy::derive_ord_xor_partial_ord)]
 impl<T: Float> Ord for NotNan<T> {
     fn cmp(&self, other: &NotNan<T>) -> Ordering {