Reconsider `can_simplify()` in `list_simplify()`

[hadley]

From

list_check_all_vectors(x, call = error_call)
can_simplify <- all(list_sizes(x) == 1L)

From #909 (comment)

[DavisVaughan]

can_simplify <- list_all_vectors(x) && all(list_sizes(x) == 1L)

this also probably works

it should work even if you allow list(scalar) since itll short circuit before getting to list_sizes(), which would otherwise error on the scalar

[DavisVaughan]

another place where list_all_vectors(x, size = 1L) would be useful

[hadley]

We can't require the input be a list of vectors because we currently assume that accumulate() silently fails to simplify in that case.