fix: use std::shared_ptr instead of std::unique_ptr for shared ressource, use operator[] instead of find_field

[lemire]

• In simdjson, we can use find_field or find_field_unordered. The operator[] is mapped to find_field_unordered. The find_field is fine, but it is used when you know the order of the keys, which is more appropriate for private JSON uses.
• I feel uneasy about having one std:: unique_ptr that gets passed as a pointer. To me, it suggests that std:: shared_ptr is more appropriate.
• The reset call on the object is fine.
• I have avoided some temporary std::string instances although it is probably not significant.