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_unix.c

@@ -47,13 +47,11 @@
 #include "nvim/types.h"
 #include "nvim/os/os.h"
 #include "nvim/os/time.h"
-#include "nvim/os/event.h"
 #include "nvim/os/input.h"
 #include "nvim/os/shell.h"
 #include "nvim/os/signal.h"
 #include "nvim/os/job.h"
 #include "nvim/msgpack_rpc/helpers.h"
-#include "nvim/msgpack_rpc/defs.h"
 
 #ifdef HAVE_STROPTS_H
 # include <stropts.h>