keeping track of the beginning and end address of all free blocks, to
make merging of adjacent blocks a breeze.  The free list is no longer
kept sorted order.

Unfortunately, this still performs poorly.  It has a higher hit rate
(with the 38 minute trace I'm using; the longer traces take too long)
than the buddy system allocator with the same arena size, it is *much*
slower, the memory usage is under 20% (!!!), and despite a roving
pointer, the allocation loop is taken an average of 692 times per
allocation.  At the end of the simulation the free list contains 1039
blocks; I presume this is a steady state approximating the average
behavior.  (There are 2280 allocated blocks at that point, roughly
confirming Knuth's "50% rule" and suggesting that the block merge code
works properly.)

I guess the next idea is separate free lists per size.