💥 Redo all higher order template functions

This does what we did for a few functions in the last few commits for
every function. We now use either the `std::invocable` concept or our
own `invocable_returning` concept wherever possible to make sure we pass
function types to these template functions, since constraint errors are
a lot more readable than template deduction errors. And instead of
having to specify the return type as a template argument, we now just
use `std::invoke_result_t<F>` instead. The VST3 message handling
functions are still using the good old `typename F` since those are
overloaded polymorphic functions. This was also a good moment to modify
`AdHocSocketHandler::send()` to allow functions returning void (this got
rid of an old fixme where we had to return some dummy value from a
function instead of just not returning anything).

src/plugin/bridges/vst3-impls/plug-view-proxy.h

@@ -136,12 +136,11 @@ class Vst3PlugViewProxyImpl : public Vst3PlugViewProxy {
      * right now.
      *
      * @see Vst3HostBridge::send_mutually_recursive_message
-     *
-     * TODO: As mentioned elsewhere, refactor these functions to use
-     *       `std::invoke_result_t`
      */
-    template <typename T, typename F>
-    T run_gui_task(F fn) {
+    template <std::invocable F>
+    std::invoke_result_t<F> run_gui_task(F&& fn) {
+        using Result = std::invoke_result_t<F>;
+
         // If `Vst3Bridge::send_mutually_recursive_message()` is currently being
         // called (because the host is calling one of `IPlugView`'s methods from
         // its UGI thread), then we'll call `fn` from that same thread.
@@ -154,8 +153,8 @@ class Vst3PlugViewProxyImpl : public Vst3PlugViewProxy {
         }
 
         if (run_loop_tasks) {
-            std::packaged_task<T()> do_call(std::move(fn));
-            std::future<T> do_call_response = do_call.get_future();
+            std::packaged_task<Result()> do_call(std::forward<F>(fn));
+            std::future<Result> do_call_response = do_call.get_future();
 
             run_loop_tasks->schedule(std::move(do_call));