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.

Empieza rastreando la implementación de Record y el manejo de sus segmentos; después, compara los registros con layout fijo y variable. Define cómo deben representarse los segmentos vacíos y los metadatos del layout antes de cambiar el código; el trabajo estará terminado cuando los llamadores puedan determinar las longitudes de los segmentos sin valores None y no tengan que distinguir entre layouts fijos y variables.