refactor(treewide): make some move ctors noexcept where appropriate

This is good practice to avoid pessimisations.
Left comments for the reasoning why ctors should be noexcept.
There are some tricky cases where we intentionally want throwing move ctors/assignments.
But those cases should really be reviewed, since some of those can be replaced
with more idiomatic copy/move-and-swap.

src/libutil/callback.hh

@@ -21,7 +21,9 @@ public:
 
     Callback(std::function<void(std::future<T>)> fun) : fun(fun) { }
 
-    Callback(Callback && callback) : fun(std::move(callback.fun))
+    // NOTE: std::function is noexcept move-constructible since C++20.
+    Callback(Callback && callback) noexcept(std::is_nothrow_move_constructible_v<decltype(fun)>)
+        : fun(std::move(callback.fun))
     {
         auto prev = callback.done.test_and_set();
         if (prev) done.test_and_set();