💥 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/common/mutual-recursion.h

@@ -74,8 +74,8 @@ class MutualRecursionHelper {
      *
      * @return The return value of `fn`.
      */
-    template <std::invocable<> F>
-    std::invoke_result_t<F> fork(F fn) {
+    template <std::invocable F>
+    std::invoke_result_t<F> fork(F&& fn) {
         using Result = std::invoke_result_t<F>;
 
         // This IO context will accept incoming calls from `handle()` and
@@ -135,7 +135,7 @@ class MutualRecursionHelper {
      *
      * @tparam F Some callable function that doesn't take any parameters.
      */
-    template <std::invocable<> F>
+    template <std::invocable F>
     std::invoke_result_t<F> handle(F&& fn) {
         // If we're not currently engaged in some mutually recursive calling
         // sequence, then we'll execute the function on this thread
@@ -154,7 +154,7 @@ class MutualRecursionHelper {
      *
      * @see handle
      */
-    template <std::invocable<> F>
+    template <std::invocable F>
     std::optional<std::invoke_result_t<F>> maybe_handle(F&& fn) {
         using Result = std::invoke_result_t<F>;