Remove llply import

[MichaelChirico]

This is one step towards removing {plyr} (long superseded) as a dependency.

llply() is particularly simple since, besides use of arguments like .progress, it's effectively the same as lapply(), e.g.

lists <- list(
  # empty lists
  list(),
  setNames(list(), character()),
  list(NULL),
  list(a = NULL),
  # typical lists
  list(1, 2),
  list(a = 1, b = 2),
  # non-list coercion
  letters,
  setNames(letters, LETTERS),
  # list of non-atomic
  list(as.name('a')),
  list(b = as.name('c')),
  list(iris),
  list(b = iris)
)

for (l in lists) stopifnot(identical(plyr::llply(l, length), lapply(l, length)))