Require Sync to implement Send for Cow. Fixes maciejhirsz#37

Currently it is possible to send across references to non-sync types across thread boundaries as long as they are wrapped in a Cow. This changes the trait bounds for `Send` to be the same as they are for the standard library's Cow https://doc.rust-lang.org/std/borrow/enum.Cow.html#impl-Send

src/generic.rs

@@ -527,5 +527,6 @@ where
     }
 }
 
+// Safety: Same bounds as `std::borrow::Cow`.
 unsafe impl<T: Beef + Sync + ?Sized, U: Capacity> Sync for Cow<'_, T, U> {}
-unsafe impl<T: Beef + Send + ?Sized, U: Capacity> Send for Cow<'_, T, U> {}
+unsafe impl<T: Beef + Sync + ?Sized, U: Capacity> Send for Cow<'_, T, U> {}