'main': Optimize the unquoted arguments characterwise walker

Fixes #730.

% repeat 3 { zsh -f tests/test-zprof.zsh main | tee … | grep -w _zsh_highlight | head -n1 }
19)    1       26787.11 26787.11  100.00%      5.33     5.33    0.02%  _zsh_highlight
19)    1       26900.64 26900.64  100.00%      5.45     5.45    0.02%  _zsh_highlight
19)    1       26915.76 26915.76  100.00%      5.47     5.47    0.02%  _zsh_highlight

Merge remote-tracking branch 'danielsh/perf-argument-1-v2'

* danielsh/perf-argument-1-v2:
  'main': Further optimize argument parsing.
  'main': Optimize a hot path.
  tests: Add a performance testing script, for measuring the performance of the 'main' highlighter on a large file.

highlighters/main/main-highlighter.zsh

@@ -1311,8 +1311,12 @@ _zsh_highlight_main_highlighter_highlight_argument()
       fi
   esac
 
-  for (( ; i <= $#arg ; i += 1 )); do
+  # This loop is a hot path.  Keep it fast!
+  (( --i ))
+  while (( ++i <= $#arg )); do
+    i=${arg[(ib.i.)[\\\'\"\`\$\<\>\*\?]]}
     case "$arg[$i]" in
+      "") break;;
       "\\") (( i += 1 )); continue;;
       "'")
         _zsh_highlight_main_highlighter_highlight_single_quote $i