feat(store): support cosmos-sdk proof specs

[rnbguy]

We need to generalize our store implementation so that it can support:

• tendermint_spec
• iavl_spec

Differences:

• Prepending during hashing: This is easily solvable by applying a closure during hashing at each node.
• Children size: Our nodes contain values. And the concatenation during hashing happens as following - left, value, right. Whereas the tendermint or iavl spec contains values only at leaves.