Create chk_all_na()

[StefanoMezzini]

Added chk_all_na(), which checks all(is.na(x) using the standard chk_*() and vld_*() syntax. I've:

• added tests based on the tests from chk_not_any_na(),
• checked for 100% test coverage,
• expanded the corresponding vignette (chk-families.Rmd), and
• updated the docs.

[StefanoMezzini]

I wrote this for use in check_data() when we expect entire columns to be NA (e.g., no factor levels in values, all factor levels in values are NA).

[joethorley]

great