Minor warnings related to tidyr > 1.0.0 compatibility

[hawkrobe]

After tidyr version 1.0.0, this warning is thrown:

`cols` is now required.
Please use `cols = c(strap)`

There is also now a tibble deprecation warning:

`as_data_frame()` is deprecated as of tibble 2.0.0.
Please use `as_tibble()` instead.
The signature and semantics have changed, see `?as_tibble`.

[cjhook]

It looks like the 'cols is now required' warning is from unnest():

https://tidyr.tidyverse.org/reference/nest_legacy.html

[hawkrobe]

see also the profiling in #8 (where the updated unnest() is also the source of the slowdown).

[dyurovsky]

fixed by 082557d