ENH: primitiveEntry construct with empty token list

- contents to be filled with a later assignment.

  This can help avoid deduction issues when populating from a list of
  tokens, and the templated version is selected:

  ```
  template<class T>
  primitiveEntry(const keyType&, const T&);
  ```

ENH: support construct empty ITstream with given name

- populate contents later by assigment