Helper functions in StatBlock and SpellSheet

[flashgnash]

StatBlock, SpellSheet, and their shared trait could do with some more functionality

• Property called "json" which contains the serde Value (serialized at construction), so that functions using one of these structs don't need to keep serializing jsonified_message every time
• Property for StatBlock called Stats, which is a hashmap of the "stats" property in the json
• Property for SpellSheet called Spells, which is a hashmap of the spell block json

[flashgnash]

get_json() implemented