Optimize class assignments

[SebKrantz]

It turns out that calling replacement functions in base R entails a deep copy in some cases:

# No deep copy
f2 <- function(x) {
  oldClass(x) <- "dfd"
  x
}

# Deep copy
f3 <- function(x) `oldClass<-`(x, "dfd")

collapse still has unnecessary deep copies in some places.