driver: Widget binding: Use ${(k)widgets} instead of $(zle -la)

Avoids a fork.

zsh-syntax-highlighting.zsh

@@ -236,7 +236,7 @@ _zsh_highlight_bind_widgets()
 
   # Override ZLE widgets to make them invoke _zsh_highlight.
   local cur_widget
-  for cur_widget in ${${(f)"$(builtin zle -la)"}:#(.*|orig-*|run-help|which-command|beep|set-local-history|yank)}; do
+  for cur_widget in ${${(k)widgets}:#(.*|orig-*|run-help|which-command|beep|set-local-history|yank)}; do
     case $widgets[$cur_widget] in
 
       # Already rebound event: do nothing.