`vec_cast.list.default()` should work row wise on data frames

[DavisVaughan]

This currently changes the size of the result because [[ is used in vec_cast.list.default()

vctrs::vec_cast(data.frame(x = 1:2), list())
#> [[1]]
#> [1] 1 2

I think this is a case where we should use vec_get(), which might be implemented to extract a single row from a data frame. (See #626)

In the mean time, we should special case data frames in vec_cast.list.default().

This breaks the following, which is how I found it.

vctrs::vec_c(data.frame(x = 1:2), .ptype = list())
#> Error in vctrs::vec_c(data.frame(x = 1:2), .ptype = list()): Internal error in `vec_assign()`: `value` should have been recycled to fit `x`.