Request: Use `BTreeMap` instead of `HashMap` for serialization

[double-daruma]

Since HashMap is used for the stats field in serialization for the habit_record.json file the entries in the json file are in an arbitrary order, which changes earch time the file is written. If BTreeMap was used the entries would be sorted. This would be beneficial if somebody (like me) wants to version control their habit_record.json file as well as editing by hand. Would this be possible or are there technical reasons not to?