💥 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.h

@@ -165,7 +165,7 @@ class Vst3PluginBridge : PluginBridge<Vst3Sockets<std::jthread>> {
      *
      * @see Vst3PlugViewProxyImpl::run_gui_task
      */
-    template <typename F>
+    template <std::invocable F>
     std::optional<std::invoke_result_t<F>> maybe_run_on_mutual_recursion_thread(
         F&& fn) {
         return mutual_recursion.maybe_handle(std::forward<F>(fn));