event: Remove EventType enum and pass a callback to event_push

This approach is more flexible because we don't need to support a fixed set of
"event types", any module can push events to be handled in main loop by simply
passing a callback to the Event structure.

src/nvim/os/event.c

@@ -170,19 +170,7 @@ static size_t process_from(klist_t(Event) *queue)
   Event event;
 
   while (kl_shift(Event, queue, &event) == 0) {
-    switch (event.type) {
-      case kEventSignal:
-        signal_handle(event);
-        break;
-      case kEventRStreamData:
-        rstream_read_event(event);
-        break;
-      case kEventJobExit:
-        job_exit_event(event);
-        break;
-      default:
-        abort();
-    }
+    event.handler(event);
     count++;
   }
 

src/nvim/os/event_defs.h

@@ -7,16 +7,12 @@
 #include "nvim/os/rstream_defs.h"
 
 typedef void * EventSource;
+typedef struct event Event;
+typedef void (*event_handler)(Event event);
 
-typedef enum {
-  kEventSignal,
-  kEventRStreamData,
-  kEventJobExit
-} EventType;
-
-typedef struct {
+struct event {
   EventSource source;
-  EventType type;
+  event_handler handler;
   union {
     int signum;
     struct {
@@ -25,6 +21,6 @@ typedef struct {
     } rstream;
     Job *job;
   } data;
-} Event;
+}; 
 
 #endif  // NVIM_OS_EVENT_DEFS_H

src/nvim/os/job.c

@@ -489,7 +489,7 @@ static void emit_exit_event(Job *job)
 {
   Event event = {
     .source = job_event_source(job),
-    .type = kEventJobExit,
+    .handler = job_exit_event,
     .data.job = job
   };
   event_push(event);

src/nvim/os/rstream.c

@@ -430,7 +430,7 @@ static void emit_read_event(RStream *rstream, bool eof)
 {
   Event event = {
     .source = rstream_event_source(rstream),
-    .type = kEventRStreamData,
+    .handler = rstream_read_event,
     .data.rstream = {
       .ptr = rstream,
       .eof = eof

src/nvim/os/signal.c

@@ -164,7 +164,7 @@ static void signal_cb(uv_signal_t *handle, int signum)
 
   Event event = {
     .source = signal_event_source(),
-    .type = kEventSignal,
+    .handler = signal_handle,
     .data = {
       .signum = signum
     }