Changed Format design.

As mentioned in last design, Format’s two instance variables — field
and builder — didn’t go together — they stood apart.  To fix this, we
converted fields to a local variable.  To match this change, we
modified other classes.

NOTE - Format now looks more cohesive, but its print methods stand out
— they are not using any of Format’s instance variables.  This
indicates that these print methods belong elsewhere.  We’ll explore
this later.