💥 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/common.h

@@ -51,14 +51,13 @@ class PluginBridge {
      *   Using a lambda here feels wrong, but I can't think of a better
      *   solution right now.
      *
-     * @tparam F A `TSockets(boost::asio::io_context&, const PluginInfo&)`
-     *   function to create the `TSockets` instance.
-     *
      * @throw std::runtime_error Thrown when the Wine plugin host could not be
      *   found, or if it could not locate and load a VST3 module.
      */
-    template <typename F>
-    PluginBridge(PluginType plugin_type, F create_socket_instance)
+    template <invocable_returning<TSockets,
+                                  boost::asio::io_context&,
+                                  const PluginInfo&> F>
+    PluginBridge(PluginType plugin_type, F&& create_socket_instance)
         // This is still correct for VST3 plugins because we can configure an
         // entire directory (the module's bundle) at once
         : config(load_config_for(get_this_file_location())),