ENH: cleanup of NamedEnum

- Remove the unused enums() method since it delivers wholly unreliable
  results. It is not guaranteed to cover the full enumeration range,
  but only the listed names.

- Remove the unused strings() method.
  Duplicated functionality of the words(), but was never used.

- Change access of words() method from static to object.
  Better code isolation. Permits the constructor to take over
  as the single point of failure for bad input.

- Add values() method

- do not expose internal (HashTable) lookup since it makes it more
  difficult to enforce constness and the implementation detail should
  not be exposed. However leave toc() and sortedToc() for the interface.

STYLE: relocated NamedEnum under primitives (was containers)

- internal typedef as 'value_type' for some consistency with STL conventions