Simplify condition + remove workaround + require R 3.2

[olivroy]

Great work on #167 !

I just wanted to send this to fine tune changes in #166

• Including some simplifications of conditions (requires R 3.2 for anyNA())
• Rename the vignette to labelled.Rmd to get a Get Started icon on the website (add a redirect)
• Improve testing infra by requiring memisc to be installed in checks (currently, skipping tests) as pak doesn't install Enhances by default. Correction. It seems it is installed on the coverage action, but not on the check.
• Removing workaround for dplyr::where() since it is now exported
• .data is already imported from rlang
• Address a tidyselect deprecation warning. (an external selection must be wrapped with all_of().
• Remove partial matching in tests
• Use across() and where() in example

[larmarange]

Thank you very much