plugin: Add hook registration

I might have gone a bit overboard with the type-checking, but
typesafe_cb_cast is quite nice to use, so why not. The macro to
register a new hook encapsulates the entire flow from param
serialization, to dispatch, parsing and callback dispatch in one
bundle. I was tempted to have the callback outside of the
registration, but it's unlikely that we'll have two calls to the same
hook with different callbacks.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

lightningd/plugin_hook.c

@@ -0,0 +1,6 @@
+#include <lightningd/plugin_hook.h>
+
+void plugin_hook_call_(struct lightningd *ld, const struct plugin_hook *hook,
+		       void *payload, void *cb_arg)
+{
+}