Implement pivot_longer()

[markfairbanks]

A few notes:

• I added vec_cast() to add_dt_wrappers() - I wasn't sure if there was a different way you'd rather add that.
• I implemented a version of pmap_chr() taken from rlang's compat-purrr.R file for use in str_extract() to avoid a purrr dependency.

[hadley]

Looks great, thanks! A few minor comments/questions below.

[mgirlich]

In hindsight of making tidyr::pivot_longer_spec() and tidyr::build_longer_spec() generics (see tidyverse/tidyr#1101) it would probably make sense to internally split this function into dtplyr_pivot_longer_spec() and dtplyr_build_longer_spec(). This makes it easier to convert them to the corresponding generics as soon as they are converted to S3 methods. The same holds true for pivot_wider()

[hadley]

When I document this locally, I don't get the \method{} syntax in the .Rd files? Are you doing something special to make this work?

[markfairbanks]

I was actually running into the same thing when I just tried, but it looks like you just have to run library(tidyr). I must have had tidyr loaded in the session to do some tests when I was documenting them originally.

[hadley]

Ah of course; and that never happens for me because I always run in a clean session.

[hadley]

Used a hack from @gaborcsardi to always attach tidyr when documenting.

[hadley]

Thanks!