[Persistence] Update pocket/smt to have support for a Merkle Sum Trie

[Olshansk]

Objective

Update the SMT to (optionally) support "Merkle Sum" tree behaviour

Origin Document

1. Relay Chains & GeoZones presentation
2. Relay Mining Document
3. Plasma Core Merkle Sum Tree

Goals

• Enable the core utility of M3 in pocket-v1

Deliverable

• Add a configuration that enables the SMT to behave as a Merkle sum tree per Plasma Core's specification
• Tests to prove the integrity of the new changes
• Benchmarks to show the performance impact
• Update the documentation where needed

Non-goals / Non-deliverables

• Improve the performance of the SMT (memory and compute-wise)
• Integrate the changes into pocket-core/pocket

---

Creator: @Olshansk
Co-Owners: @h5law

[h5law]

Completed in #845