Rewrite :defer semantics

:defer now supports a hook, a cons cell with (SYMBOL . INTEGER) where
SYMBOL is a hook and INTEGER is a number of idle seconds before the
package is autoloaded, or just the integer (as per the default behavior
of :defer).

Also fixes an issue where switch-buffer-deffered packages (like
smartparens) wouldn't load.

modules/completion/ivy/config.el

@@ -24,7 +24,7 @@ immediately runs it on the current candidate (ending the ivy session)."
 ;;
 
 (def-package! ivy
-  :defer input
+  :defer (pre-command-hook . 1)
   :config
   (setq ivy-height 12
         ivy-do-completion-in-region nil