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/shell_spec.lua

@@ -14,7 +14,7 @@ local helpers = require('test.unit.helpers')
 local shell = helpers.cimport(
   './src/nvim/os/shell.h',
   './src/nvim/option_defs.h',
-  './src/nvim/os/event.h',
+  './src/nvim/main.h',
   './src/nvim/misc1.h'
 )
 local ffi, eq, neq = helpers.ffi, helpers.eq, helpers.neq