Module shielded

Provides high-level access to database shielded:

• nullifiers
• note commitment trees
• anchors

This module makes sure that:

• all disk writes happen inside a RocksDB transaction, and
• format-specific invariants are maintained.

Correctness

crate::constants::state_database_format_version_in_code() must be incremented each time the database format (column, serialization, etc) changes.