Switch interrupt_handler to return maybe_t<char_event_t>

Prepares to remove R_NULL

src/input.cpp

@@ -249,7 +249,7 @@ void input_mapping_add(const wchar_t *sequence, const wchar_t *command, const wc
 
 /// Handle interruptions to key reading by reaping finshed jobs and propagating the interrupt to the
 /// reader.
-static maybe_t<int> interrupt_handler() {
+static maybe_t<char_event_t> interrupt_handler() {
     // Fire any pending events.
     event_fire_delayed();
     // Reap stray processes, including printing exit status messages.
@@ -260,10 +260,10 @@ static maybe_t<int> interrupt_handler() {
         if (vintr == 0) {
             return none();
         }
-        return vintr;
+        return char_event_t{vintr};
     }
 
-    return R_NULL;
+    return char_event_t{R_NULL};
 }
 
 static std::atomic<bool> input_initialized{false};

src/input_common.h

@@ -124,7 +124,7 @@ class char_event_t {
 
 /// A type of function invoked on interrupt.
 /// \return the event which is to be returned to the reader loop, or none if VINTR is 0.
-using interrupt_func_t = maybe_t<int> (*)();
+using interrupt_func_t = maybe_t<char_event_t> (*)();
 
 /// Init the library with an interrupt function.
 void input_common_init(interrupt_func_t func);