Genode: add scheduleCallbacks to asyncdispatch (#20708)

* Genode: add native signal handler

* Genode: add scheduleCallbacks to asyncdispatch

This resolves some awkwardness where an RPC server may or may not
use callSoon while dispatching RPC but without scheduling timers
or I/O.

lib/genode/constructibles.nim

@@ -0,0 +1,21 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2022 Emery Hemingway
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+type Constructible*[T] {.
+  importcpp: "Genode::Constructible",
+  header: "<util/reconstructible.h>", byref, pure.} = object
+
+proc construct*[T](x: Constructible[T]) {.importcpp.}
+  ## Construct a constructible C++ object.
+
+proc destruct*[T](x: Constructible[T]) {.importcpp.}
+  ## Destruct a constructible C++ object.
+
+proc constructed*[T](x: Constructible[T]): bool {.importcpp.}
+  ## Test if an object is constructed.