`[[.vctrs_rcrd` should not allow selections of size >1

[DavisVaughan]

https://github.com/r-lib/vctrs/blob/657ace5ca7ddeda123c1d661eb9bf88ce84d75d7/R/type-rcrd.R#L86-L90

> x <- new_rcrd(list(x = 1, y = 2))
> unclass(x[[c(1, 1)]])
$x
[1] 1 1
$y
[1] 2 2

Should probably be using vec_as_location2() somewhere? Or vec_slice2()?