Closes #2012

Both feisr and merTools break with dev vctrs. They both try to call list_unchop() with a list array.

Internally we assume that if obj_is_list() is true, then r_length(x) == vec_size(x) and are interchangeable. This is what broke list_unchop() for these two packages. We switched to vec_size() instead of r_length() in the C code after a call to obj_check_list()

It's not clear what list_unchop() should even do with a list array. It's also not clear for functions like list_drop_empty(), where the output is unconditionally an atomic list vector. I'd argue that most vctrs list_*() functions that check their input with obj_is_list() are not prepared for list arrays, so obj_is_list() should simply return FALSE on them.

So the example from #2012 no longer silently succeeds with an incorrect answer, but instead returns a clear failure of:

library(vctrs)

xs <- list(
  data.frame(x = 1:2, y = 1:2),
  data.frame(x = 3:4, y = 3:4),
  data.frame(x = 4:5, y = 4:5)
)
dim(xs) <- c(1, 3)

dplyr::bind_rows(xs)
#> Error in `list_flatten()`:
#> ! `x` must be a list, not a list matrix.