[Storage][JellyfishMerkle] Reorder a few code blocks
[Storage][JellyfishMerkle] Reorder a few code blocks

I always find it hard to look for code in this file. Putting the `impl`
blocks closer to the struct definition so things are structured more
consistently.

Closes: #1726
Approved by: lightmark

aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra