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/finally.hh

@@ -20,7 +20,11 @@ public:
     // Copying Finallys is definitely not a good idea and will cause them to be
     // called twice.
     Finally(Finally &other) = delete;
-    Finally(Finally &&other) : fun(std::move(other.fun)) {
+    // NOTE: Move constructor can be nothrow if the callable type is itself nothrow
+    // move-constructible.
+    Finally(Finally && other) noexcept(std::is_nothrow_move_constructible_v<Fn>)
+        : fun(std::move(other.fun))
+    {
         other.movedFrom = true;
     }
     ~Finally() noexcept(false)