Major design changes in Exporters package.

1. In base.Exporter, reduced attribute methods (an eye sore) --
level(), number(), code(), etc., -- to just field() and level().
2. PartBuilder (previously PartSchema) part() now passes both attribute
& value, making the change in 1 possible.
3. Previous PartSchema now split into 3 objects -- PartSchema,
Attribute, and PartBuilder.
4.  PartSchema mimics an Enum.
5. PartBuilder now does the assembly of the part from part attributes
(of course working with Exporter).
6. Exporter methods and variables renamed for clarity.
7. Related changes elsewhere.