DESIGN: `clone` / `inplace` argument

[sebffischer]

When marshaling objects that have reference semantics, having a clone / inplace parameter for the (un)marshal generics might be handy.

The pseudocode below illustrates a call to marshal(), where cloning is not necessary and another call to unmarshal() where it is necessary.

g <- function() {
  x_marshaled <- callr::r({
    x <- f(...)
    marshal(x, clone = FALSE)
  })
  x_unmarshaled <- unmarshal(x_marshaled, clone = TRUE)
  y <- h(x_unmarshaled)
  return(list(x_marshaled, y))
}

To stay on the safe side, marshal methods for objects with reference semantics should always clone by default and not modify th object that is being marshaled in-place. Because marshal() if often called right before sending the object to another process, it might be worth to optimize the special case where in-place modifications are allowed (or in general, the object that is being marshaled is not being further used).