Improve dcast guessing?

[MichaelChirico]

Current guess behavior:

function(x) {
  if ("value" %chin% names(x))
    return("value")
  if ("(all)" %chin% names(x))
    return("(all)")
  var <- names(x)[ncol(x)]
  message("Using '", var, "' as value column. Use 'value.var' to override")
  return(var)
}

1. Look for column value
2. Look for column (all)
3. Return the last column

When this was created, value.var didn't support multiple values: https://github.com/Rdatatable/data.table/blob/e95b9be0c31113d4a398c58b6231baf8fd7bb4e8/R/fcast.R

At a glance, it seems to me a strictly better version would be to use setdiff(names(x), all.var(formula))

This would be a potentially breaking change, so gathering feedback first.