data_out function to dynamically allocate return value.

data_out function to dynamically allocate return value.

Preparation for i18n of values.  Instead of asking the
caller to prepare the buffer for output, data_out now
dynamically allocates the output value, and expects the
caller to free it.  This is necessary since for utf8
strings, the caller cannot reasonably know the length of
the required output buffer.  It also simplifies some uses
of data_out.