'rbind', 'cbind' not detected as S3 generics for export tag

[kevinushey]

E.g.

#' @export
rbind.foobar <- function(..., deparse.level = 1) {
  print("hello")
}

roxygen2 doesn't detect that rbind is an S3 generic, and so emits export(rbind.foobar) rather than S3method(rbind,foobar).

[kevinushey]

This occurs because roxygen2:::is_s3_generic hides its S3 method check in is.primitive, but rbind is an internal function, not a primitive:

https://github.com/klutometis/roxygen/blob/master/R/s3.R#L22-L26

I think that bit needs to accept both primitive functions and 'internal' functions (perhaps some special casing is needed for rbind, cbind?)