Avoid KEEP() and FREE() in vec_shaped_ptype()

[DavisVaughan]

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#2044"
  ),
  fn = \() {
    library(vctrs)
    x <- as.list(1:1e6)
    bench::mark(vec_ptype_common(!!!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/v… vec_ptype… 41.1ms 41.7ms      24.0    7.63MB     5.99   160    40      6.68s <int> 
2 r-lib/v… vec_ptype… 35.1ms 35.7ms      27.7    7.63MB     6.93   160    40      5.77s <int> 
3 r-lib/v… vec_ptype… 28.9ms 29.5ms      33.5    7.63MB     8.37   160    40      4.78s <int> 
# ℹ 3 more variables: memory <list>, time <list>, gc <list>