ObjFW  Check-in [28170f5f65]

A linked-list is put before each memory chunk allocated instead of
having an array of all memory chunks. This means only one malloc now
instead of one malloc and one realloc. This also means that when
checking reallocs and frees, it's no longer necessary to iterate through
all memory chunks, as the linked list also contains the owner, meaning
realloc and free are no longer O(n), but O(1) now.

As allocating bigger chunks seems to be a little bit slower than smaller
chunks, it seems that this is slightly slower in benchmarks if only very small
chunks are allocated. However, measuring real world usage, it's a lot faster.