reduce the number "Undefined global functions or variables"

This part of `devtools::check()` output is really useful to flag unqualified function calls. However, as it currently outputs a list of 100+ names, it isn't practical to use it if you only care about the new code. It would be great if this list was cut down considerably.

- Make sure that at least exported functions have a test that can help check that the output hasn't changed.
- Use `codetools::checkUsagePackage('blabr')` to list the usages with the file paths and line numbers.
- Most of the names come from using tidy selection or data masking in tidyverse verbs. For those, using the `.data` "pronoun" should do it.