Add get-string-n' and get-string-n!' for R6RS ports

* libguile/r6rs-ports.c (scm_get_string_n_x): Implement `get-string-n!'
  in C for efficiency.
* libguile/r6rs-ports.h: Add prototype for this function.
* module/ice-9/binary-ports.scm: Export `get-string-n!'.

* module/rnrs/io/ports.scm (get-string-n): Implement based on
  `get-string-n!'.
  Export both `get-string-n!' and `get-string-n'.
* module/rnrs.scm: Also export these.

* test-suite/tests/r6rs-ports.test (8.2.9 Textual input): Add a few
  tests for `get-string-n' and `get-string-n!'.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

module/ice-9/binary-ports.scm

@@ -37,6 +37,7 @@
             get-bytevector-n!
             get-bytevector-some
             get-bytevector-all
+            get-string-n!
             put-u8
             put-bytevector
             open-bytevector-output-port

module/rnrs.scm

@@ -182,7 +182,8 @@
           make-custom-textual-output-port
           call-with-string-output-port
 	  flush-output-port put-string
-          get-char get-datum get-line get-string-all lookahead-char
+          get-char get-datum get-line get-string-all get-string-n get-string-n!
+          lookahead-char
           put-char put-datum put-string
           standard-input-port standard-output-port standard-error-port
           

module/rnrs/io/ports.scm

@@ -68,8 +68,9 @@
           put-u8 put-bytevector
 
           ;; textual input
-          get-char get-datum get-line get-string-all lookahead-char
-           
+          get-char get-datum get-line get-string-all get-string-n get-string-n!
+          lookahead-char
+
           ;; textual output
           put-char put-datum put-string
 
@@ -386,6 +387,17 @@ return the characters accumulated in that port."
 (define (get-string-all port)
   (with-i/o-decoding-error (read-delimited "" port 'concat)))
 
+(define (get-string-n port count)
+  "Read up to @var{count} characters from @var{port}.
+If no characters could be read before encountering the end of file,
+return the end-of-file object, otherwise return a string containing
+the characters read."
+  (let* ((s (make-string count))
+         (rv (get-string-n! port s 0 count)))
+    (cond ((eof-object? rv) rv)
+          ((= rv count)     s)
+          (else             (substring/shared s 0 rv)))))
+
 (define (lookahead-char port)
   (with-i/o-decoding-error (peek-char port)))