More data types
This implements all property datatypes, and Lexemes. Notes
- Refactored much of entity.rs into one file per entity type
- Refactored much of from_json.rs into the appropriate structs
- Added (de)serialization traits
- Entity is now just a wrapper around the individual types
Tests all run through. If you have code depending on from_json functions, it might need updating, otherwise everything should work as before.