R7RS: Add \| escape inside strings.

Author: ecraven

LOG

@@ -1138,3 +1138,4 @@
   it isn't an attractive hazzard.
     configure, makefiles/Mf-install.in
 - add #\null and #\escape as aliases for #\nul and #\esc (R7RS compatibility)
+- allow \| escape inside strings (R7RS compatibility)

mats/6.ms

@@ -1574,6 +1574,7 @@
   (eqv? (string-ref "\\\"\'" 0) #\\)
   (eqv? (string-ref "\\\"\'" 1) #\")
   (eqv? (string-ref "\\\"\'" 2) #\')
+  (eqv? (string-ref "\|\v" 0) #\|)
   (= (char->integer (string-ref "a\012" 1)) #o12 10)
   (= (char->integer (string-ref "a\015" 1)) #o15 13)
   (= (char->integer (string-ref "a\177" 1)) #o177 127)

s/read.ss

@@ -778,7 +778,7 @@
          (with-read-char c
            (state-case c
              [eof (with-unread-char c (xcall rd-eof-error "string"))]
-             [(#\\ #\")
+             [(#\\ #\" #\|)
               (with-stretch-buffer i c
                 (*state rd-token-string (fx+ i 1)))]
              [(#\n #\a #\b #\f #\r #\t #\v)