Inline r_pairlist_find()

[DavisVaughan]

Pushed upstream in r-lib/rlang#1830

Used by r_dim(), r_class(), etc, which is in our deepest loops (size-common, ptype-common), so we want these helpers to be fully inlined.

In particular r_dim() was slower than Rf_getAttrib(x, R_DimSymbol) and would show up in Instruments profiling, and I don't want to have to decide between the two.

cross::bench_versions(
  pkgs = c(
    # Main before doing any of these PRs
    "r-lib/vctrs@0279470c6de6ee2db3a9b333c198754080554ee7",
    # Current main
    "r-lib/vctrs",
    # This PR
    "r-lib/vctrs#2039"
  ),
  fn = \() {
    library(vctrs)
    x <- as.list(1:1e6)
    bench::mark(list_unchop(x), iterations = 200)
  }
)

# A tibble: 3 × 14
  pkg           expression   min median `itr/sec` mem_alloc `gc/sec` n_itr  n_gc total_time result
  <chr>         <bch:expr> <bch> <bch:>     <dbl> <bch:byt>    <dbl> <int> <dbl>   <bch:tm> <list>
1 r-lib/vctrs@… list_unch… 150ms  158ms      6.17    11.4MB     10.2   200   332      32.4s <int> 
2 r-lib/vctrs   list_unch… 141ms  150ms      6.52    11.4MB     10.8   200   332      30.7s <int> 
3 r-lib/vctrs#… list_unch… 140ms  147ms      6.64    11.4MB     11.0   200   332      30.1s <int> 
# ℹ 3 more variables: memory <list>, time <list>, gc <list>