ENH: improve consistency of adding tokens to ITstream

- use add_tokens() instead of the old multi-parameter
  append(.., bool) method which was misleading since it added tokens
  at the current tokenIndex, not at the end.

- stringify ITstream contents with CharStream instead of StringStream.
  Allows string_view for copying out the content.

ENH: set namedDictionary dictionary name from Istream

- provides context for error messages etc (#2990)