[Rd] RFC: R_data_class as part of the C-API

[Laurent Gautier]

Hi,

In src/attrib.c, the comment for the function R_data_class is:

```
/* the S4-style class: for dispatch required to be a single string;
    for the new class() function;
    if(!singleString) , keeps S3-style multiple classes.
    Called from the methods package, so exposed.
  */
SEXP R_data_class(SEXP obj, Rboolean singleString)
```

I am obviously writing this because I'd wish to see it in the API,
and I think that the above (a package shipped with R does
need this as part of the API) is a pretty good case for having it.

Best,

Laurent

PS: I have found a similar request made some time in the past (one
of the points in
https://stat.ethz.ch/pipermail/r-devel/2007-October/047070.html
).