Allow creating another constructor (without validation) with new_empty_object()

[eitsupi]

Close #522

Allows us to create a constructor separately from the S7 class definition.
The expected use is to create an empty (invalid) object when the package is loaded, and then the second constructor sets properties on it and creates a valid object.

It is also possible for the first constructor of the S7 class to depend on the second constructor.

The second constructor may completely bypass the validation step, making object creation much faster, which I think is very important in packages like polars where we create objects multiple times.
(It is entirely up to the package developer to ensure that the second constructor creates a valid object without validation.)

library(S7)

new_sums <- function(x) {
  out <- EMPTY_SUMS
  value <- x |> Reduce(f = `+`) |> as.integer()
  prop(out, "value", check = FALSE) <- value
  out
}

Sums <- new_class(
  "Sums",
  properties = list(value = class_integer),
  constructor = function(...) {
    v <- new_sums(list(...))@value
    new_object(
      S7_object(),
      value = v
    )
  }
)

EMPTY_SUMS <- new_empty_object(Sums)

bench::mark(
  sums1 = Sums(1, 2, 3),
  sums2 = new_sums(list(1, 2, 3))
)
#> # A tibble: 2 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 sums1       68.81µs     80µs    11654.   39.27KB     19.0
#> 2 sums2        4.92µs   6.29µs   143152.    4.14MB     28.6

^{Created on 2025-11-26 with reprex v2.1.1}