Attempt metadata overhaul

[mnadareski]

This is a major overhaul of Metadata-related classes and namespaces. Notable changes include:

• Concrete types and enums where appropriate to avoid frequent and repeated conversion
• Removal of DictionaryBase, ModelBackedItem, and ModelBackedItem<T> types
• Consolidation of extensions and enums to appropriate namespaces
• Near-complete removal of Reflection-based methods
• Major fixes to the filtering framework, including allowing alternate field names again
• Major increases in test coverage, especially around filtering
• Addition of RomVault-specific header item