This PR makes a few adjustment on the Bitstring Status List implementation, by first renaming the wrongly named bitstream_status_list module into bitstring_status_list. But also:

• make the fields of BitstringStatusListEntry public and add a constructor,
• simplify serialization of the type property using serde's tag attribute instead of a singleton type BitstringStatusListEntryType,
• same for BitstringStatusListType; and
• add constructors and methods for StatusPurpose.