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/shell.c

@@ -8,7 +8,7 @@
 #include "nvim/ascii.h"
 #include "nvim/lib/kvec.h"
 #include "nvim/log.h"
-#include "nvim/os/event.h"
+#include "nvim/event/loop.h"
 #include "nvim/os/job.h"
 #include "nvim/os/rstream.h"
 #include "nvim/os/shell.h"