Recommend using all_of() for looking up variables

[lionel-]

First step towards #76. For now this is a simple message:

vars <- letters[1:4]
vars_select(letters, vars)
#> Note: Selecting non-column variables is brittle.
#> ℹ If the data contains `vars` it will be selected instead.
#> ℹ Use `all_of(vars)` to silence this message.
#>   a   b   c   d
#> "a" "b" "c" "d"

We can move on to formally deprecating this kind of lookup in the next version.

I would like to refer to all_of() in the message, but there will be a transition period where dplyr hasn't exported the new selectors. To work around this, the second commit binds the selection helpers inside the context mask. This is what several client packages are already doing. This makes it easier to use dplyr functions without the package attached:

# Before
dplyr::select(mtcars, dplyr::starts_with("c"))

# After
dplyr::select(mtcars, starts_with("c"))

It is still recommended to export the helpers so their help topics are easily accessible.