loop: Simplify loop.c and move some code to input.c

- Declare poll timer in Loop structure instead of a loop_poll_events local
  variable.
- Move deferred event management to input.c

src/nvim/edit.c

@@ -601,11 +601,11 @@ edit (
      * Get a character for Insert mode.  Ignore K_IGNORE.
      */
     lastc = c;                          /* remember previous char for CTRL-D */
-    loop_enable_deferred_events(&loop);
+    input_enable_events();
     do {
       c = safe_vgetc();
     } while (c == K_IGNORE);
-    loop_disable_deferred_events(&loop);
+    input_disable_events();
 
     if (c == K_EVENT) {
       c = lastc;