driver: Make it reentrant.

This fixes an infinite recursion in zsh without zle-line-pre-redraw [≤5.2] in
the following situation:

% source zsh-syntax-highlighting.zsh
% eval "my-self-insert() { zle -M 'foobar'; ${(q)widgets[self-insert]#*:} \"\$@\" }"
% zle -N self-insert my-self-insert
% source zsh-syntax-highlighting.zsh

Fixes sorin-ionescu#305.

zsh-syntax-highlighting.zsh

@@ -249,6 +249,7 @@ _zsh_highlight_call_widget()
 _zsh_highlight_bind_widgets()
 {
   setopt localoptions noksharrays
+  local prefix=orig-s$SECONDS-r$RANDOM # unique each time, in case we're sourced more than once
 
   # Load ZSH module zsh/zleparameter, needed to override user defined widgets.
   zmodload zsh/zleparameter 2>/dev/null || {
@@ -258,7 +259,7 @@ _zsh_highlight_bind_widgets()
 
   # Override ZLE widgets to make them invoke _zsh_highlight.
   local -U widgets_to_bind
-  widgets_to_bind=(${${(k)widgets}:#(.*|orig-*|run-help|which-command|beep|set-local-history|yank)})
+  widgets_to_bind=(${${(k)widgets}:#(.*|run-help|which-command|beep|set-local-history|yank)})
 
   # Always wrap special zle-line-finish widget. This is needed to decide if the
   # current line ends and special highlighting logic needs to be applied.
@@ -283,13 +284,13 @@ _zsh_highlight_bind_widgets()
       # NO_function_argzero, regardless of the option's setting here.
 
       # User defined widget: override and rebind old one with prefix "orig-".
-      user:*) zle -N orig-$cur_widget ${widgets[$cur_widget]#*:}
-              eval "_zsh_highlight_widget_${(q)cur_widget}() { _zsh_highlight_call_widget orig-${(q)cur_widget} -- \"\$@\" }"
+      user:*) zle -N $prefix-$cur_widget ${widgets[$cur_widget]#*:}
+              eval "_zsh_highlight_widget_${(q)cur_widget}() { _zsh_highlight_call_widget ${(q)prefix}-${(q)cur_widget} -- \"\$@\" }"
               zle -N $cur_widget _zsh_highlight_widget_$cur_widget;;
 
       # Completion widget: override and rebind old one with prefix "orig-".
-      completion:*) zle -C orig-$cur_widget ${${(s.:.)widgets[$cur_widget]}[2,3]} 
-                    eval "_zsh_highlight_widget_${(q)cur_widget}() { _zsh_highlight_call_widget orig-${(q)cur_widget} -- \"\$@\" }"
+      completion:*) zle -C $prefix-$cur_widget ${${(s.:.)widgets[$cur_widget]}[2,3]} 
+                    eval "_zsh_highlight_widget_${(q)cur_widget}() { _zsh_highlight_call_widget ${(q)prefix}-${(q)cur_widget} -- \"\$@\" }"
                     zle -N $cur_widget _zsh_highlight_widget_$cur_widget;;
 
       # Builtin widget: override and make it call the builtin ".widget".