imp(testkit): adopt basecoin-store in ibc-testkit

[rnbguy]

Feature Summary

Currently, ibc-testkit uses a MockContext with a very minimalistic data structure. We want to improve this to capture more adversarial scenarios.

To achieve this, along with other refactors, we must introduce a blockchain-compatible store like Merkle Tree.

basecoin already has a working Merkle tree with a helpful set of store data structures and types. We want to move the store crate from there under ibc-testkit. We may directly use it when it is published on crates.io.

Proposal

<= 0.49.1

[rnbguy]

closed by #1109