Turn padding bytes into a freelist?
- In combination with skipped tail padding of structs, this could
  potentially save some memory in applications that allocate
  small chunks of memory (e.g. small strings, short, int, etc) in addition
  to data that demands padding (such as structs)

                      +---------------+  +---------------+
                      v               |  v               |
  header allocation1 pad allocation2 padpad allocation3 backbyte ...
                                      ^  |               ^
                                      +--+               |
                                                      free_ptr