Implement AsRef instead of custom method

Clippy emits a warning since we define a method that has the same name
as a standard trait. Implement the trait `AsRef` instead of using a
custom method.

secp256k1-sys/src/macros.rs

@@ -34,13 +34,6 @@ macro_rules! impl_array_newtype {
                 dat.as_mut_ptr()
             }
 
-            #[inline]
-            /// Gets a reference to the underlying array
-            pub fn as_ref(&self) -> &[$ty; $len] {
-                let &$thing(ref dat) = self;
-                dat
-            }
-
             #[inline]
             /// Returns the length of the object as an array
             pub fn len(&self) -> usize { $len }
@@ -50,6 +43,15 @@ macro_rules! impl_array_newtype {
             pub fn is_empty(&self) -> bool { false }
         }
 
+        impl AsRef<[$ty; $len]> for $thing {
+            #[inline]
+            /// Gets a reference to the underlying array
+            fn as_ref(&self) -> &[$ty; $len] {
+                let &$thing(ref dat) = self;
+                dat
+            }
+        }
+
         impl PartialEq for $thing {
             #[inline]
             fn eq(&self, other: &$thing) -> bool {