add scm_{to,from}_{utf8,latin1}_string{n,}

add scm_{to,from}_{utf8,latin1}_string{n,}

* libguile/strings.h:
* libguile/strings.c (scm_from_latin1_string, scm_to_latin1_string): New
  functions, in terms of the latin1_stringn variants.
  (scm_from_utf8_string, scm_from_utf8_stringn)
  (scm_to_utf8_string, scm_to_utf8_stringn): New functions.
  (scm_i_from_utf8_string, scm_i_to_utf8_string): Removed these internal
  functions.
  (scm_from_stringn): Handle -1 as a length. Unlike the previous
  behavior of scm_from_locale_string (NULL), which returned the empty
  string, we now raise an error.  The null pointer is not the same as
  the empty string.

* libguile/stime.c (scm_strftime, scm_strptime): Adapt to publishing of
  utf8 functions.