Lack of support for nullable types

PHP 7.1 and above has nullable types.

Parser doesn`t recognize them.

Example:
Change the example function declaration at the bottom of demo.php and add nullable type:

function foo(?array $a) {

Effect:

PHP Fatal error: Uncaught RuntimeException: Unknown Expr Type NullableType in .../vendor/ircmaxell/php-cfg/lib/PHPCfg/Parser.php:799
Stack trace:
#0 .../vendor/ircmaxell/php-cfg/lib/PHPCfg/Parser.php(1253): PHPCfg\Parser->parseExprNode(Object(PhpParser\Node\NullableType))
#1 .../vendor/ircmaxell/php-cfg/lib/PHPCfg/Parser.php(95): PHPCfg\Parser->parseParameterList(Object(PHPCfg\Func), Array)
#2 .../vendor/ircmaxell/php-cfg/lib/PHPCfg/Parser.php(332): PHPCfg\Parser->parseFunc(Object(PHPCfg\Func), Array, Array, NULL)
#3 .../vendor/ircmaxell/php-cfg/lib/PHPCfg/Parser.php(146): PHPCfg\Parser->parseStmt_Function(Object(PhpParser\Node\Stmt\Function_))
#4 .../vendor/ircmaxell/php-cfg/lib/PHPCfg/Parser.php(132): PHPCfg\Parser->parseNode(Object(PhpParser\Node\Stmt\Function_))
#5 .../vendor/ircmaxell/php-cfg/lib/PHPCfg/Parser.php(100): PHPCfg\Parser-> in .../vendor/ircmaxell/php-cfg/lib/PHPCfg/Parser.php on line 799

Start in lib/PHPCfg/Parser.php around the reported failure at line 799, then reproduce the issue by updating the example function declaration in demo.php to use ?array. Done means the parser accepts that nullable type without the reported Unknown Expr Type NullableType fatal when demo.php is run.