brackets: Optimize the character iteration

Test case: See the first comment of issue #295.

Before:
num  calls                time                       self            name
-----------------------------------------------------------------------------------
 1)   14         284.67    20.33   82.84%    284.67    20.33   82.84%  _zsh_highlight_highlighter_brackets_paint
 2)   15         313.72    20.91   91.30%     26.62     1.77    7.75%  _zsh_highlight
 3)   14         318.68    22.76   92.74%      7.41     0.53    2.16%  _zsh_highlight_call_widget

After:
num  calls                time                       self            name
-----------------------------------------------------------------------------------
 1)   15          57.25     3.82   45.33%     28.30     1.89   22.41%  _zsh_highlight
 2)   14          81.96     5.85   64.90%     27.02     1.93   21.40%  _zsh_highlight_call_widget
 3)   14          26.31     1.88   20.84%     26.31     1.88   20.84%  _zsh_highlight_highlighter_brackets_paint
 4)    2          10.66     5.33    8.44%      8.97     4.49    7.10%  VCS_INFO_formats
 5)    2          25.92    12.96   20.52%      7.72     3.86    6.11%  VCS_INFO_get_data_git

(That's not in my everyday configuration but in my minimal testing
setup, which runs vcs_info with the default settings.)

changelog.md

@@ -112,6 +112,8 @@ to issue #418.
   revision zsh-5.8-172-gdd6e702ee or newer is also fine.)
   [#418, https://github.com/okapia/zsh-viexchange/issues/1]
 
+- Improve performance of the `brackets` highlighter.
+
 
 # Changes in version 0.7.1
 

highlighters/brackets/brackets-highlighter.zsh

@@ -51,8 +51,9 @@ _zsh_highlight_highlighter_brackets_paint()
   local -A levelpos lastoflevel matching
 
   # Find all brackets and remember which one is matching
-  for (( pos = 1; pos <= buflen; pos++ )) ; do
-    char=$BUFFER[pos]
+  pos=0
+  for char in ${(s..)BUFFER} ; do
+    (( ++pos ))
     case $char in
       ["([{"])
         levelpos[$pos]=$((++level))