chunk +-----------------------------------------------+
      | Size of previous chunk (if P = 0)             |
      +-------------------------------------------+---+
      +-------------------------------------+---+ | P |
      | Size of this chunk                  | 0 | +---+
      +-------------------------------------+---+-----+
      | Pointer to next chunk in bin list             |
      +-----------------------------------------------+
      | Pointer to previous chunk in bin list         |
      +-----------------------------------------------+
      | Pointer to left child when in bin trie        |
      +-----------------------------------------------+
      | Pointer to right child when in bin trie       |
      +-----------------------------------------------+
      | Pointer to parent when in bin trie            |
      +-----------------------------------------------+
      | Bin index when in bin trie                    |
      +-----------------------------------------------+
      |                                               |
      : Free                                          :
      |                                               |
chunk +-----------------------------------------------+
      | Size of this chunk again                      |
      +-------------------------------------------+---+
      +-------------------------------------+---+ | 1 |
      | Size of next chunk                  | U | +---+
data  +-------------------------------------+---+-----+

// Adjusted from dlmalloc source code comments.