[R6RS] Library syntax versioning

[dyb at cs.indiana.edu]

We already have (as of last week's meeting) a syntax for dealing with
implementation-dependent lexical exceptions, which is to allow for
#!<symbol-like-thing>, e.g.:

  #!mzsceheme
  #!larceny
  ...

Perhaps we can plan on using the same tool for future extensions to the
syntax:

  #!r7rs

We can even require #!r6rs to appear at the top of a library now, or at
least allow it to be included.

This is a lot more concise than a MIME content-type line.

Kent