Use 32-bit integers for redblack_id_t

On 32-bit systems, the shape cache size is 1048576 (value of
REDBLACK_CACHE_SIZE), but a 16-bit unsigned integer can only go up to
65536. This means that the redblack_id_t can overflow and lead to a
corrupted red-black tree.

The following script crashes on 32-bit systems:

    o = Object.new
    1_000_000.times do |i|
      o.instance_variable_set(:"@i#{i}", i)
    end

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