Simplify extraction of formals from primitive functions
Simplify extraction of formals from primitive functions

As mentioned in `?formals` the formals of primitive functions can be
retrieved with `formals(args(.))`.

The previous behavior caused an unfortunate interaction between
RStudio's completion engine and the [lookup
package](https://github.com/jimhester/lookup), which uses a custom
`print.function` when **lookup** is attached, and was being
inadvertently called during argument completion.

Fixes https://github.com/jimhester/lookup/issues/14

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