[R6RS] Library syntax versioning

[dyb at cs.indiana.edu]

> It hardly matters what #!r6rs represents, but I further
> propose that it be an external representation for the
> canonical unspecified value we voted into existence last
> week; that value has no external representation as yet,
> so no one can say #!r6rs is redundant.

So that people don't start writing #!r6rs when they want to return the
unspecified value and implementations don't start printing #!r6rs for the
unspecified value, I propose instead that read instead treat #!r6rs
essentially like a comment, so that, e.g., a single call to read from a
file containing

  #!r6rs 17

returns 17.

Kent