Highlight using XHPAST in parallel

Summary:
We usually try to highlight two files at once in Differential:
https://secure.phabricator.com/diffusion/P/browse/master/src/applications/differential/parser/DifferentialChangesetParser.php;3706ec4879a1250c$602-603

Test Plan:
  $highlighter = new PhutilXHPASTSyntaxHighlighter();
  $futures = array(
    $highlighter->getHighlightFuture(file_get_contents('../src/__phutil_library_map__.php')),
    $highlighter->getHighlightFuture(file_get_contents('../src/__phutil_library_map__.php')),
  );

  Futures($futures)->resolveAll();

Saw drop from 0.424 s to 0.380 s.

Also:

  <?php
  $highlighter = new PhutilXHPASTSyntaxHighlighter();
  $futures = array(
    $highlighter->getHighlightFuture('echo "a";'),
    $highlighter->getHighlightFuture('echo "b";'),
    $highlighter->getHighlightFuture('syntax error'),
    $highlighter->getHighlightFuture('<?php'),
    $highlighter->getHighlightFuture('<?php syntax error'),
  );

  foreach (Futures($futures) as $future) {
    echo $future->resolve()."\n\n\n";
  }

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4874