[FR] Allow rfun to optionally run in parallel over iters

Looking at the code for rfun and looking at the future dev guide I think that that we could allow users to optionally specify whether the rfun should be allowed to run over iterations in parallel.

plain(multicore)
my_fun = function(x, y) {
  return x + y
}
# parallel_args is a list of args for `future.mapply`
my_fun_rvar = rfun(my_fun, c("x", "y"), parallel = TRUE, parallel_args = list(...))

plain(multicore)
x = # ... 
y = # ...
z = my_fun_rvar(x, y)

I think we would just have to modify the signature like

rfun <- function (.f, rvar_args = NULL, ndraws = NULL, parallel = FALSE, parallel_args = list()) 

Then in the code when we call mapply we call future.mapply. I think for the parallel_args we would need to wrap future.mapply in a do.call as well but I can't remember rn

Start in R/rvar-rfun.R at rfun and inspect the existing mapply call. Read the linked future package developer guide before determining how future.mapply and parallel_args fit the function interface. Done means rfun can optionally run over iterations in parallel while retaining the current serial behavior.