Use list2() in vec_recycle_common() and avoid allocations when possible

[DavisVaughan]

Two wins:

• Using list2() to avoid the allocation that came with rlang_env_dots_list()
• That still wasn't enough, because vec_recycle_common() was cloning as well, even in the no-op case! So I switched it to be smart enough to return the object unchanged if no recycling is required for any element.

pkgs <- tibble::tribble(
  ~vctrs             , ~rlang        ,
  "vctrs"            , "rlang"       ,
  "r-lib/vctrs#2070" , "r-lib/rlang" ,
)

# Just no-op recycle
cross::bench_versions(pkgs = pkgs, {
  library(vctrs)
  x <- as.list(1:1e6)
  bench::mark(vec_recycle_common(!!!x, .size = 1))
})
#> # A tibble: 2 × 7
#>   pkg                      expression    min median `itr/sec` mem_alloc `gc/sec`
#>   <chr>                    <bch:expr> <bch:> <bch:>     <dbl> <bch:byt>    <dbl>
#> 1 vctrs, rlang             vec_recyc…   38ms   38ms      26.3    7.63MB     316.
#> 2 r-lib/vctrs#2070, r-lib… vec_recyc… 10.9ms 11.3ms      87.1    3.06KB       0

# No-op recycle and size determination
cross::bench_versions(pkgs = pkgs, {
  library(vctrs)
  x <- as.list(1:1e6)
  bench::mark(vec_recycle_common(!!!x))
})
#> # A tibble: 2 × 7
#>   pkg                    expression     min  median `itr/sec` mem_alloc `gc/sec`
#>   <chr>                  <bch:expr> <bch:t> <bch:t>     <dbl> <bch:byt>    <dbl>
#> 1 vctrs, rlang           vec_recyc… 146.1ms 150.1ms      6.39    7.63MB     27.1
#> 2 r-lib/vctrs#2070, r-l… vec_recyc…  23.9ms  25.1ms     39.7     3.27KB      0

# Recycle to known size
cross::bench_versions(pkgs = pkgs, {
  library(vctrs)
  x <- as.list(1:1e6)
  x <- c(x, list(1:2))
  bench::mark(vec_recycle_common(!!!x, .size = 2), iterations = 10)
})
#> # A tibble: 2 × 7
#>   pkg                    expression     min  median `itr/sec` mem_alloc `gc/sec`
#>   <chr>                  <bch:expr> <bch:t> <bch:t>     <dbl> <bch:byt>    <dbl>
#> 1 vctrs, rlang           vec_recyc… 182.9ms 346.4ms      2.65    7.63MB     6.36
#> 2 r-lib/vctrs#2070, r-l… vec_recyc…  94.3ms  95.3ms      7.87    7.63MB     9.44

# Recycle and size determination
cross::bench_versions(pkgs = pkgs, {
  library(vctrs)
  x <- as.list(1:1e6)
  x <- c(x, list(1:2))
  bench::mark(vec_recycle_common(!!!x), iterations = 10)
})
#> # A tibble: 2 × 7
#>   pkg                       expression   min median `itr/sec` mem_alloc `gc/sec`
#>   <chr>                     <bch:expr> <bch> <bch:>     <dbl> <bch:byt>    <dbl>
#> 1 vctrs, rlang              vec_recyc… 273ms  492ms      2.09    7.63MB     9.40
#> 2 r-lib/vctrs#2070, r-lib/… vec_recyc… 109ms  113ms      6.86    7.63MB     8.24