Modify bytevectors/string conversions to allow wide strings

Modify bytevectors/string conversions to allow wide strings

The bytevector to string conversion functions were accomplished
by converting via locale strings.  This did not allow conversions
of wide strings in an 8-bit locale.  This is avoided by using knowledge
of the storage format of the string.

* libguile/bytevectors.c (STRING_TO_UTF, scm_string_to_utf8): modify
  string to bytevector conversion to use internal string information
  (UTF_TO_STRING, scm_utf8_to_string): modify bytevector to string
  conversion