ENH: align Enum methods with HashTable

- deprecate get(key, deflt) in favour of lookup(key, deflt).
  Method name compatibility with HashTable.

- deprecate operator().
  The meaning is too opaque and equally served by other means:

  - use get(key) instead of operator()(key).
    Const access whereas HashTable::operator()(key)
    creates missing entry.

  - lookup(key, deflt) - instead of operator()(key, deflt).
    Const access whereas HashTable::operator()(key, deflt)
    creates a missing entry.

- make Enum iterable to allow participation in range-for etc.