[wpiutil] uv: use move for std::function (#3653)

Also use function_ref for Loop::Walk().

wpiutil/src/main/native/include/wpi/uv/Loop.h

@@ -15,6 +15,7 @@
 #include <utility>
 
 #include "wpi/Signal.h"
+#include "wpi/function_ref.h"
 #include "wpi/uv/Error.h"
 
 namespace wpi::uv {
@@ -171,7 +172,7 @@ class Loop final : public std::enable_shared_from_this<Loop> {
    *
    * @param callback A function to be invoked once for each active handle.
    */
-  void Walk(std::function<void(Handle&)> callback);
+  void Walk(function_ref<void(Handle&)> callback);
 
   /**
    * Reinitialize any kernel state necessary in the child process after