[Rd] exists, get and get0 accept silently inputs of length > 1

[Antoine Fabri]

Dear R-devel,

The doc of exists, get and get0 is unambiguous, x should be an object given
as a character string. However these accept longer inputs. It can lead an
uncareful user to think these functions are vectorized when they're not,
and generally lets through bugs that one might have preferred to trigger
earlier failure.

``` r
exists("d")
#> [1] FALSE
exists(c("c", "d"))
#> [1] TRUE
get(c("c", "d"))
#> function (...)  .Primitive("c")
get0(c("c", "d"))
#> function (...)  .Primitive("c")
```

I believe these should either fail, or be vectorized, probably the former.

Thanks,

Antoine

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