ScriptFu: TinyScheme: add call-with-output-string etc to dialect

Add canonical/idiomatic functions call-with-output-string,
call-with-input-string, any->string to the library init.scm.
Since they are useful, especially in test scripts and frameworks.

Remove their definitions from test scripts and test frameworks.

Also comment out failing test in test script string-escape.scm.

plug-ins/script-fu/scripts/script-fu.init

@@ -668,6 +668,34 @@
                          (set-output-port prev-outport)
                          res)))))
 
+
+; Idioms using string-port.
+; See MIT/GNU Scheme.
+; Analogs of functions using file-port.
+; Used string-ports are not closed, they go out of scope and are garbage collected.
+
+; Returns string that procedure outputs to a port.
+; Require procedure takes a port.
+; Procedure result is lost, only its side effects on the port are returned as a string.
+(define (call-with-output-string procedure)
+  (let ((port (open-output-string )))
+    (procedure port)
+    (get-output-string port)))
+
+
+; Returns the result of calling procedure.
+; Require procedure takes a port.
+; The port passed to the procedure is a string-port from the given string.
+(define (call-with-input-string str procedure)
+  (let ((port (open-input-string str)))
+    (procedure port)))
+
+; Returns string representation of any Scheme object
+; aka MIT write-to-string
+(define (any->string any)
+  (call-with-output-string (lambda (port) (write any port))))
+
+
 ; Random number generator (maximum cycle)
 (define *seed* 1)
 (define (random-next)