String ports use UTF-8; ignore %default-port-encoding.

* libguile/strports.c (scm_mkstrport): Use UTF-8; ignore
  %default-port-encoding.  Rename 'str_len' and 'c_pos' to
  'num_bytes' and 'c_byte_pos'.  Interpret 'pos' argument
  as a character index instead of a byte index.

* module/ice-9/boot-9.scm (%cond-expand-features): Add srfi-6 to the
  list of core features.

* module/srfi/srfi-6.scm (open-input-string, open-output-string): Simply
  re-export these, since the core versions are now compliant.

* doc/ref/api-io.texi (String Ports): Remove text that describes
  non-compliant behavior of string ports with regard to encoding.

* doc/ref/srfi-modules.texi (SRFI-0): Add srfi-6 to the list of
  core features.
  (SRFI-6): Remove text that mentions non-compliant behavior of
  core string ports.

* module/ice-9/format.scm (format):
* module/ice-9/pretty-print.scm (truncated-print):
* module/rnrs/io/ports.scm (open-string-input-port,
  open-string-output-port):
* test-suite/test-suite/lib.scm (format-test-name):
* test-suite/tests/chars.test ("combining accent is pretty-printed",
  "combining X is pretty-printed"):
* test-suite/tests/ecmascript.test (eread, eread/1):
* test-suite/tests/rdelim.test:
* test-suite/tests/reader.test (read-string):
* test-suite/tests/regexp.test:
* test-suite/tests/srfi-105.test (read-string): Don't set
  %default-port-encoding before creating string ports.

* benchmark-suite/benchmarks/ports.bm (%latin1-port): Use
  'set-port-encoding!' to set the string port encoding.
  (%utf8/ascii-port, %utf8/wide-port, "rdelim"): Don't set
  %default-port-encoding before creating string ports.

* test-suite/tests/r6rs-ports.test ("lookahead-u8 non-ASCII"): Don't set
  %default-port-encoding before creating string ports.
  ("put-bytevector with UTF-16 string port", "put-bytevector with
  wrong-encoding string port"): Use 'set-port-encoding!' to set the
  string port encoding.

* test-suite/tests/print.test (tprint): Use 'set-port-encoding!' to set
  the string port encoding.
  ("truncated-print"): Use 'pass-if-equal'.

* test-suite/tests/ports.test ("encoding failure leads to exception",
  "%default-port-encoding is honored", "peek-char [latin-1]", "peek-char
  [utf-8]", "peek-char [utf-16]"): Remove tests.
  ("%default-port-encoding is ignored", "peek-char"): Add tests.
  ("suitable encoding [latin-1]", "suitable encoding [latin-3]",
  "wrong encoding, error", "wrong encoding, substitute",
  "wrong encoding, escape"): Use 'set-port-encoding!' to set the
  string port encoding.
  ("%default-port-encoding, wrong encoding"): Rewrite to use
  a file port instead of a string port.

benchmark-suite/benchmarks/ports.bm

@@ -1,6 +1,6 @@
 ;;; ports.bm --- Port I/O.         -*- mode: scheme; coding: utf-8; -*-
 ;;;
-;;; Copyright (C) 2010, 2011, 2012 Free Software Foundation, Inc.
+;;; Copyright (C) 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
 ;;;
 ;;; This program is free software; you can redistribute it and/or
 ;;; modify it under the terms of the GNU Lesser General Public License
@@ -34,16 +34,15 @@
   (string-concatenate (make-list (* iteration-factor 10000) s)))
 
 (define %latin1-port
-  (with-fluids ((%default-port-encoding #f))
-    (open-input-string (large-string "hello, world"))))
+  (let ((p (open-input-string (large-string "hello, world"))))
+    (set-port-encoding! p "ISO-8859-1")
+    p))
 
 (define %utf8/ascii-port
-  (with-fluids ((%default-port-encoding "UTF-8"))
-    (open-input-string (large-string "hello, world"))))
+  (open-input-string (large-string "hello, world")))
 
 (define %utf8/wide-port
-  (with-fluids ((%default-port-encoding "UTF-8"))
-    (open-input-string (large-string "안녕하세요"))))
+  (open-input-string (large-string "안녕하세요")))
 
 
 (with-benchmark-prefix "peek-char"
@@ -87,6 +86,5 @@
 
   (let ((str (string-concatenate (make-list 1000 "one line\n"))))
     (benchmark "read-line" 1000
-               (let ((port (with-fluids ((%default-port-encoding "UTF-8"))
-                             (open-input-string str))))
+               (let ((port (open-input-string str)))
                  (sequence (read-line port) 1000)))))