Bulletproof allocator

[jwise]

Would be nice for the allocator to become a little more bulletproof. Specifically, it would be good to have:

• Corruption checking (per-arena cookies -- check for buffer overruns, and check to make sure that memory is returned to the heap it was allocated from)
• Allocator heap integrity check function
• Allocator heap size used diagnostics (i.e., memory leak tracking)

[ginge]

Additionally

• high water mark for this arena (i.e., max memory used this session. reset on app reload)