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/helm/config.el

@@ -11,7 +11,7 @@
 ;;
 
 (def-package! helm-mode
-  :defer (input . 1)
+  :defer (pre-command-hook . 1)
   :config
   (helm-mode +1)
   ;; helm is too heavy for find-file-at-point