Selecting verbs (e.g. `select()` or verbs supporting `vars()`
selections) now treat symbols a bit differently. Symbols are only
evaluated in the data context, never in the calling context (#2904).
This means that you can no longer refer to local variables unless
you are explicitly unquoting these variables with `!!`.
Note that since last version, helper calls (like `starts_with()`)
obey the opposite behaviour and are evaluated in the calling
context, not the data context. To sum up, symbols can only refer to
data frame objects, while helpers can only refer to contextual