Reorganization: split out utilities; mapping methods

  * Move non-project-specific Python utilities into a separate file.  They
    might conceivably find their way into a new project some day.

  * Implement the standard mapping methods generically (i.e., in terms of
    basic Python mapping operations).

  * Use the mapping methods for the various key-object-related mappings.

  * Start fixing setter functions for null values.