Remove code related to vectors in packed struct

[chung-leong]

Since Zig 0.16.0 no longer allows this.

[chung-leong]

Test is failing:

 ✘ Handle struct in packed struct
   │
   │ Failed asserting that two objects are equal.
   │ --- Expected
   │ +++ Actual
   │ @@ @@
   │      'struct2' => stdClass Object (...)
   │      'number' => 200
   │      'struct3' => stdClass Object (
   │ -        'number1' => 5
   │ -        'number2' => 6
   │ +        'number1' => 20
   │ +        'number2' => 24
   │      )
   │  )
   │
   │ /home/cleong/zigar/php-zigar/test/type-handling/struct/StructHandlingTest.php:110
   │

[chung-leong]

Something is still broken:

 ✘ Handle bool in vector
   │
   │ Exception: out of bound (zig)
   │
   │ /home/cleong/zigar/php-zigar/test/type-handling/bool/BoolHandlingTest.php:199
   │