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.

test/unit/os/fs_spec.lua

@@ -19,7 +19,7 @@ require('bit')
 cimport('unistd.h')
 cimport('./src/nvim/os/shell.h')
 cimport('./src/nvim/option_defs.h')
-cimport('./src/nvim/os/event.h')
+cimport('./src/nvim/main.h')
 cimport('./src/nvim/fileio.h')
 local fs = cimport('./src/nvim/os/os.h')
 cppimport('sys/stat.h')