Refactor chunk map.

Break the chunk map into two separate arrays, in order to improve cache
locality. This is related to issue #23.

include/jemalloc/internal/size_classes.sh

@@ -240,7 +240,7 @@ cat <<EOF
  * The small_size2bin lookup table uses uint8_t to encode each bin index, so we
  * cannot support more than 256 small size classes.  Further constrain NBINS to
  * 255 since all small size classes, plus a "not small" size class must be
- * stored in 8 bits of arena_chunk_map_t's bits field.
+ * stored in 8 bits of arena_chunk_map_bits_t's bits field.
  */
 #if (NBINS > 255)
 #  error "Too many small size classes"