[bugfix] When SQL generation code was ported, there were left some loose points.
[janitor] Declare constants for the basic symbolic column types
[janitor] Removing SQL generation methods, now they are moved to BP::Loader::Storage::Relational.
[feature] Generation of JSON model
[feature] TO_JSON accessory methods have been moved from BP::Model to
BP::Loader::Storage::MongoDB , as they are only used on this specific
storage backend.