Post-order snapshot

[cool-develope]

IAVL v1 and v0 export is based on post-order, implement post-order export for compatibility with IAVL v0 and v1

[tac0turtle]

should we only do restore for post order that way its less code to maintain over the long term?

[yihuang]

#703

This compact format rely on post-order BTW.

[tac0turtle]

we should evaluate which is overall better (compact and performant). I dont think we should support both in iavlv2, only one will get used in reality. This can come later

[cool-develope]

with #853