Implement Hash trait for SmallVec (#337)

While calling the `hash` function on a `SmallVec` currently works via
dereferencing into `&[T]` (see `test_hash` on `tests.rs`), this is not
enough to allow deriving `Hash` on e.g. a struct containing a single
field of type `SmallVec`.

Author: hsanzg

src/lib.rs

@@ -69,6 +69,8 @@ use alloc::alloc::Layout;
 use core::borrow::Borrow;
 use core::borrow::BorrowMut;
 use core::fmt::Debug;
+use core::hash::{Hash, Hasher};
+use core::marker::PhantomData;
 use core::mem::align_of;
 use core::mem::size_of;
 use core::mem::ManuallyDrop;
@@ -79,8 +81,6 @@ use core::ptr::copy;
 use core::ptr::copy_nonoverlapping;
 use core::ptr::NonNull;
 
-use core::marker::PhantomData;
-
 #[cfg(feature = "serde")]
 use serde::{
     de::{Deserialize, Deserializer, SeqAccess, Visitor},
@@ -2049,6 +2049,12 @@ where
     }
 }
 
+impl<T: Hash, const N: usize> Hash for SmallVec<T, N> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.as_slice().hash(state)
+    }
+}
+
 impl<T, const N: usize> Borrow<[T]> for SmallVec<T, N> {
     #[inline]
     fn borrow(&self) -> &[T] {