Fix bugs related to mutation-sharing substrings

Fix bugs related to mutation-sharing substrings

* libguile/strings.c (scm_i_is_narrow_string, scm_i_try_narrow_string,
  scm_i_string_set_x): Check to see if the provided string is a
  mutation-sharing substring, and do the right thing in that case.
  Previously, if such a string was passed to these functions, they would
  behave very badly: while trying to fetch and/or mutate the cell
  containing the stringbuf, they were actually fetching or mutating the
  cell containing the original shared string.  That's because
  mutation-sharing substrings store the original string in CELL_1,
  whereas all other strings store the stringbuf there.