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

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>