Fixes #316.

* danielsh/i316-v2:
  'main': Enable fallback to the 'arg0' style.
  'main': Set fallback style for the 'arg0' style.

The fallback style name uses '_' in anticipation of a future auto-fallback
feature keyed on style names.

'arg0' was previously known as 'commandtypefromthefuture'.

* upstream/pr/374:
  'main': Directly count spaces to skip, don't leverage proc_buf length

Main highlighter run on itself, on the optimized version:

- optimized (8 runs, 3 last noted):
1.1201650000
1.1074430000
1.1263810000

- unoptimized (8 runs, 3 last noted):
1.5746400000
1.5115250000
1.5155440000

Average difference: 0.415907

Part of issue #139.

Fixes #344.

Fixes #96.

Running main highlighter on itself (both runs are on the optimized
version):

- clean (8 runs, last 3 noted):
1.7007670000
1.7330720000
1.7038810000

- optimized (8 runs, last 3 noted):
1.5007230000
1.5142960000
1.4973320000

Average difference: 0.208456

When parsing main-highlighter with itself, the *_check_path function is
called 426 times. Note that there are 686 region_highlight resulting
entries.

Fixes #335.

Fixes #333.

Fixes #368.

Fixes a subset of issue #237.

The heretofore code warned once per keypress.

This is related to a future "unbalanced { ( ) }" check for issue #344.

Also adds an XFail test for balanced braces for issue #344.

This was suggested on #355.

This caused warnings with `setopt warn_create_global`.

This manifested in completion of the form «./foo<TAB>» where there happened to
be a program called 'foo' in $PATH.

Fixes #354.
Closes #355.

Fixes #329.

Merge remote-tracking branch 'danielsh/i329-v1'

* danielsh/i329-v1:
  highlighters: Rename entry points.
  driver: Rename highlighter entry points

This tracks the API change made in the previous commit, as suggested in the
(#if 0'd) deprecation warning.

This updates the docs and the driver, in a manner backwards compatible with
existing highlighters.  (None of the highlighters are touched by this change,
yet tests continue to pass.)

Part of issue #329.

Re-fixes #305.

This is a prerequisite for the next commit.

The incumbent code was wrong: the test of $in_redirection in the first hunk
would never have seen «(( in_redirection == 2 ))».  That had no visible
effect since options to sudo don't look like redirection operators.

The structure now mirrors the stall construct at the top of the loop.

Part of issue #305.

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 #305.