Better representation of segments in multi-segment records

The current implementation is rather fragile and hard to generalize at times. In particular:

• The segments attribute is a list of Record or None objects. Perhaps empty records could be represented with an empty-represented Record object instead. One cannot tell the segment length from the None object.
• For variable-layout records, the layout record is read into the first segment. Maybe there should be a way to abstract away the layout record so that applications do not need to know about fixed vs variable layout records.

Inizia tracciando l’implementazione di Record e la gestione dei suoi segmenti, quindi confronta i record con layout fisso e variabile. Definisci come devono essere rappresentati i segmenti vuoti e i metadati del layout prima di modificare il codice; il lavoro è completato quando i chiamanti possono determinare le lunghezze dei segmenti senza valori None e non devono distinguere tra layout fissi e variabili.