MVP: Create API and suitable unit tests for LMDB d(step) bindings

Motivation:

The RocksDB API is unwieldy and its C++ runtime is quite memory intensive.

In comparison, LMDB is simple (and simple to create bindings for), well-tested, has excellent performance and fits in 32KiB (L1 cache size) at runtime.

Tasks:

• Create bindings
• Implement D API via moss-db
• Create unit tests demonstrating that things work as expected