tidyr 0.8.3

• crossing() preserves factor levels (#410), now works with list-columns
  (#446, @SamanthaToet). (These also help expand() which is built on top
  of crossing())

• nest() is compatible with dplyr 0.8.0.

• spread() works when the id variable has names (#525).

• unnest() preserves column being unnested when input is zero-length (#483),
  using list_of() attribute to correctly restore columns, where possible.

• unnest() will run with named and unnamed list-columns of same length
  (@hlendway, #460).