Implement decoupled state commitments and storage

[roysc]

Introduces a new CommitKVStore to implement ADR-040 proposal of decoupled commitment/data concerns.

Adapted from store/ll-smt branch (see https://github.com/cosmos/cosmos-sdk/pull/8507).

Description

Introduces the decoupled.Store type composed of:

• A tm-db interface for mapping state storage (SS) (as key → value)
• A "state commitment" (SC) store for computing Merkle root hashes (storing hash(key) → hash(key, value))
  • This is an iavl.Store held as a CommitKVStore reference, which will later be refactored to use an SMT store
• A tm-db interface for an inverted index, mapping SC values back to SS keys (hash(key, value) → key)

closes: #1

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[roysc]

This should be a MVP state store for #1. Requires changes from the corresponding branch on tm-db: https://github.com/roysc/tm-db/tree/implement-adr-40 - (which just patches in minimal versioning support)

[roysc]

Moving this work to a PR on the main repo: https://github.com/cosmos/cosmos-sdk/pull/9485