8.7.0: Destructuring

• Fixed issue #31: Support list() reference assignment for PHP < 7.3
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• Fixed foreach statement using destructuring assignments in PHP 7.0
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• Merged PR #146: Add support for omitting expressions in destructuring
  assignments, e.g. list($a, , $b)= $expr or [, $a]= $expr.
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• Merged PR #145: Add support for specifying keys in list(), which has
  been in PHP since 7.1.0, see https://wiki.php.net/rfc/list_keys
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• Merged PR #144: Rewrite list(&$a)= $expr for PHP 7.0, 7.1 and 7.2
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