Add implementation of "transcoded ports"

Add implementation of "transcoded ports"

* libguile/r6rs-ports.c (make_tp, tp_write, tp_fill_input, tp_flush)
  (tp_close, initialize_transcoded_ports, scm_i_make_transcoded_port): New
  functions.
  (scm_init_r6rs_ports): Call `initialize_transcoded_ports'.
* module/rnrs/ports.scm (transcoded-port): Actually implement,
  using `%make-transcoded-port'.
* test-suite/tests/r6rs-ports.test ("8.2.6 Input and output ports"): Added a
  few tests for `transcoded-port'.

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