- the construction of compound tokens is now split into two stages:
    - default construct
    - read contents
  This permits a larger variety of handling.

- the new token::readCompoundToken(..) method allows for simpler
  more failsafe invocations.

- forward resize(), read() methods for compound tokens to support
  separate read and population.
  Top-level refCompoundToken() method for modify access.

ENH: split off a private readCompoundToken() method within ISstream

- this allows overloading and alternative tokenisation handling for
  derived classes