[R6RS] write/read or read/write invariance

[William D Clinger]

Oops.  I wrote:

> > What will the file foo contain?
> 
> \31;\32;\33;\34;\35;

The file's contents should instead be:

\x31;\x32;\x33;\x34;\x35;

Even uglier!

I checked in a corrected version of the Unicode SRFI.

Will