Added debug and stats heap.

These two heaps should help do stricter testing, but raise a few more
new issues.

1) Need a way to conditionally define different types for main heaps.

2) Need a cleaner way to work with all the various heap types that we
have for different use cases (e.g., main heap vs. singleton heap vs.
internal heap for data structures vs. sqlite heap).

3) Need to fix up stats heap to not use a vanilla std type.