Here's a breakdown of specification changes:

- Removed string constants; move them to other SRFIs later
   (thanks Duy and John):
   * ascii-digits
   * ascii-lower-case
   * ascii-upper-case
   * ascii-punctuation

- Added more predicates to distinguish ASCII from non-ASCII objects
   (thanks John and Shiro):
   * ascii-codepoint?
   * ascii-bytevector?
   * ascii-string?

- Added control character procedures:
   * ascii-control->display
   * ascii-display->control

- Added bracket matching procedures (thanks John):
   * ascii-open-bracket
   * ascii-close-bracket
   * ascii-mirror-bracket

- Clarified numerical limits (thanks John). It's permitted to assume
   that all integers going in and out of the procedures are fixnums.
   Except it the underlying Scheme implementation supports character
   codepoints bigger than the maximum fixnum; then the implementation
   of this SRFI must also support bignums in and out.

Most of the well-known R6RS and R7RS implementations now pass their
tests and run the examples; more to come.