Reduce memory manager fragmentation

[richardkchapman]

Expanding rows (and especially row arrays) can lead to fragmentation in the memory manager. Some ideas that were discussed to improve this:

1. Make sure we use realloc calls when we can
2. Make sure that we are prepared to expand into adjacent free blocks on realloc calls (both above and below)
3. Allocate the huge blocks at the bottom of the heap rather than the top, to increase the chances of being able to do a realloc without copying