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

shape.h

@@ -9,7 +9,6 @@
 #define SHAPE_IN_BASIC_FLAGS 1
 typedef uint32_t attr_index_t;
 typedef uint32_t shape_id_t;
-typedef uint32_t redblack_id_t;
 # define SHAPE_ID_NUM_BITS 32
 
 #else
@@ -18,11 +17,12 @@ typedef uint32_t redblack_id_t;
 #define SHAPE_IN_BASIC_FLAGS 0
 typedef uint16_t attr_index_t;
 typedef uint16_t shape_id_t;
-typedef uint16_t redblack_id_t;
 # define SHAPE_ID_NUM_BITS 16
 
 #endif
 
+typedef uint32_t redblack_id_t;
+
 #define MAX_IVARS (attr_index_t)(-1)
 
 # define SHAPE_MASK (((uintptr_t)1 << SHAPE_ID_NUM_BITS) - 1)