event loop: New abstraction layer with refactored time/signal API

- Add event loop abstraction module under src/nvim/event. The
  src/nvim/event/loop module replaces src/nvim/os/event
- Remove direct dependency on libuv signal/timer API and use the new abstraction
  instead.
- Replace all references to uv_default_loop() by &loop.uv, a new global variable
  that wraps libuv main event loop but allows the event loop functions to be
  reused in other contexts.

src/nvim/os/time.c

@@ -7,7 +7,7 @@
 #include <uv.h>
 
 #include "nvim/os/time.h"
-#include "nvim/os/event.h"
+#include "nvim/event/loop.h"
 #include "nvim/vim.h"
 
 static uv_mutex_t delay_mutex;
@@ -43,7 +43,7 @@ void os_delay(uint64_t milliseconds, bool ignoreinput)
     if (milliseconds > INT_MAX) {
       milliseconds = INT_MAX;
     }
-    event_poll_until((int)milliseconds, got_int);
+    LOOP_POLL_EVENTS_UNTIL(&loop, (int)milliseconds, got_int);
   } else {
     os_microdelay(milliseconds * 1000);
   }