Make 'string-append' more efficient and robust.

Make 'string-append' more efficient and robust.

* libguile/strings.c (scm_string_append): Check for numerical overflow
  while computing the length of the result.  Double-check that we don't
  overflow the result string, and that it is the correct length in the
  end (in case another thread changed the list).  When copying a narrow
  string to a wide result, avoid calling 'scm_i_string_length' and
  'scm_i_string_chars' on each character.