The intent is to manage long-vs-single-frame allocations better.
Previously long allocations were map-long bump allocations, and couldn't be freed
mid-map, as there was neither a reference to the allocated range, nor a
way to actully free it.
Add a two-mode block allocator (similar to previous debuffer alloc) that
allows making long and once allocations. But now long allocations are
backed by "pool" allocator and return references to the range.
This commit doesn't do the deallocation yet, so map chaning doesn't yet
work.
Ivan Avdeev