* flower/include/std-string.hh: String/std::string compatibility:

* flower/include/std-string.hh: String/std::string compatibility:
Use NPOS for `not found' (iso -1), use ssize for length () and pos
type.

* flower/rational.cc:
* flower/include/rational.hh:

* flower/offset.cc:
* flower/include/offset.hh:

* flower/interval.cc:
* flower/include/interval.hh:

* flower/string-convert.cc:
* flower/include/string-convert.hh: Use std::string [interface].
Update callers.