Improve `tspec()` performance

[mgirlich]

It is relatively slow to construct a spec.

library(tibblify)

create_spec <- function() {
  tspec_df(
    tib_int("instance_id"),
    tib_chr("date_added"),
    tib_row(
      "basic_information",
      tib_df(
        "labels",
        tib_chr("name"),
        tib_chr("entity_type"),
        tib_chr("catno"),
        tib_chr("resource_url"),
        tib_int("id"),
        tib_chr("entity_type_name"),
      ),
      tib_int("year"),
      tib_chr("master_url"),
      tib_df(
        "artists",
        tib_chr("join"),
        tib_chr("name"),
        tib_chr("anv"),
        tib_chr("tracks"),
        tib_chr("role"),
        tib_chr("resource_url"),
        tib_int("id"),
      ),
      tib_int("id"),
      tib_chr("thumb"),
      tib_chr("title"),
      tib_df(
        "formats",
        # tib_chr_vec(
        #   "descriptions",
        #   required = FALSE,
        #   input_form = "scalar_list",
        # ),
        tib_chr("text", required = FALSE),
        tib_chr("name"),
        tib_chr("qty"),
      ),
      tib_chr("cover_image"),
      tib_chr("resource_url"),
      tib_int("master_id"),
    ),
    tib_int("id"),
    tib_int("rating"),
  )
}

bench::mark(create_spec())

# Before
#> # A tibble: 1 × 6
#>   expression         min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>    <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 create_spec()   8.17ms   9.87ms      95.6     839KB     14.3

# After
#> # A tibble: 1 × 6
#>   expression         min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>    <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 create_spec()   1.33ms   1.56ms      590.     334KB     12.8

^{Created on 2022-11-28 with reprex v2.0.2}