[CM] Delimited strings in s7

[bil at ccrma.Stanford.EDU]

The reader tokenizes before it tries to make sense of a # sequence, so
#&( will be a missing-close-paren error long before you get to the
*#readers* handlers.  For most other cases you might be able to use
port-position which is settable:

(set! *#readers*
       (cons
        (cons #\& (lambda (str)
		   (if (= (string-length str) 1) ; #& followed by a reader delimiter
		       (read)
		       (begin
			 (set! (port-position (current-input-port))
			       (- (port-position (current-input-port))
				  (string-length str)
				  -1))
			 (string (string-ref str 1))))))
        ()))

(with-input-from-string "#&N"
   (lambda ()
     (display (read)) (newline)
     (display (read)) (newline)))

N
#<eof>

(with-input-from-string "#&Newton"
   (lambda ()
     (display (read)) (newline)
     (display (read)) (newline)
     (display (read)) (newline)))

N
ewton
#<eof>

(with-input-from-string "#&N#&123"
   (lambda ()
     (display (read)) (newline)
     (display (read)) (newline)
     (display (read)) (newline)
     (display (read)) (newline)))

N
1
23
#<eof>

(with-input-from-string "#&(123)"
   (lambda ()
     (display (read)) (newline)
     (display (read)) (newline)))

(123)
#<eof>

I don't want the arity of the *#reader* function to be a flag meaning
"read one character" -- this would only have one use, would require
hand-waving in the documentation, and would make error checking less
reliable.