A memory pool allocator, designed for systems that need to allocate/free
pointers in amortized O(1) time. Memory is allocated a page at a time,
then added to a set of pools of equally sized regions. A free list for
each size is maintained in the unused regions. When a pointer is
repooled, it is put at the head of the appropriate free list.

Allocations larger than mp->max_pool (configurable, usually over 2048
bytes) are allocated whole via mmap and freed immediately via munmap; no
free list is used.

For example usage, see test.c.