3.0.0:
    - add db-keys, db-values and db-pairs: previously these were available by customizing db-steam via keyword parameters
    - remove db-stream, use db-keys, db-values or db-pairs instead
2.0.0:
    - make-level now expects three arguments (implemenation name, interface implementation, resource)
      and returns a level record
    - all write operations should now return #<unspecified> instead of #t
    - added db-get/default procedure (and level-get/default method to interface)
    - interface method names now use a "level-" prefix, eg level-get instead of get
    - db-stream should now return key+value combinations as pairs instead of lists
      eg, (("key" . "value")) instead of (("key" "value"))