Don't use `new_list_of`

[sebffischer]

Instead, we should just have something like:

A <- new_class("A",
  properties = list(
    a = list_of(B)
  )
)

i.e., we don't have explicit types for the list, but list_of will just create an S7 property. This will make it easier to work with, because we don't always have to unpack these objects with @items. See https://github.com/lawremi/wizrd/blob/0615782ae88942cd622291685daf3759bf80d19f/R/utils.R#L129-L131 or RConsortium/S7#433