This implements the reduced fragmentation from the heap frontend, by
carving smaller blocks out of larger allocated blocks.

The super block and each sub-block are all flagged with BLOCK_FLAG_LFH.

The super-block (struct group) uses a standard struct block header, as
well as a list entry to be linked in free list, and a free bit map to
track free sub-blocks.

Sub-blocks reference their super block through the base_offset, instead
of the subheap, using the block size as radix.