Refresh coalesce() on top of vctrs::vec_case_when()

[DavisVaughan]

Five goals:

• Significantly speed up and reduce memory of coalesce() using vctrs::vec_case_when()
• Fix the error generated by coalesce(1, NULL, "x"), where the reported index was off due to compacting out NULL too early
• Give a better error when all inputs are NULL
• Get rid of usage of early-naming via names_as_error_names(), which is no longer required with the C impl of vec_case_when() (it falls out naturally when you rewrite in C)
• Improve on consistency regarding the output names generated by coalesce()

# Most common case
cross::bench_branches(\() {
  library(dplyr)
  set.seed(123)
  x <- sample(c(1, NA, 2), size = 1e7, replace = TRUE)
  bench::mark(coalesce(x, 0))
})
#> # A tibble: 2 × 7
#>   branch                    expression          min   median `itr/sec` mem_alloc `gc/sec`
#>   <chr>                     <bch:expr>     <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 feature/coalesce-on-vctrs coalesce(x, 0)   24.5ms   29.9ms     27.4      162MB     37.1
#> 2 main                      coalesce(x, 0)    227ms    235ms      4.26     611MB     12.8

# Missings in `y`
cross::bench_branches(\() {
  library(dplyr)
  set.seed(123)
  x <- sample(c(1, NA, 2), size = 1e7, replace = TRUE)
  y <- sample(c(1, NA, 2), size = 1e7, replace = TRUE)
  bench::mark(coalesce(x, y))
})
#> # A tibble: 2 × 7
#>   branch                    expression          min   median `itr/sec` mem_alloc `gc/sec`
#>   <chr>                     <bch:expr>     <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 feature/coalesce-on-vctrs coalesce(x, y)   30.4ms   34.4ms     25.6      229MB     41.4
#> 2 main                      coalesce(x, y)  209.1ms    211ms      4.63     572MB     18.5

# No missings in `y`
cross::bench_branches(\() {
  library(dplyr)
  set.seed(123)
  x <- sample(c(1, NA, 2), size = 1e7, replace = TRUE)
  y <- sample(c(1, 2), size = 1e7, replace = TRUE)
  bench::mark(coalesce(x, y))
})
#> # A tibble: 2 × 7
#>   branch                    expression          min   median `itr/sec` mem_alloc `gc/sec`
#>   <chr>                     <bch:expr>     <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 feature/coalesce-on-vctrs coalesce(x, y)   25.5ms   26.1ms     35.4      162MB    165. 
#> 2 main                      coalesce(x, y)  223.7ms  226.3ms      4.30     534MB     17.2

I'm happy that I can explain where all of these remaining allocations come from:

# - 1 alloc for `vec_detect_missing` (40000048, lgl vec)
# - 1 alloc for `!` of the `vec_detect_missing` result (40000048, lgl vec)
# - 1 alloc for <double> output (80000048, dbl vec)
# - 1 alloc for determining where `0` goes (10000048, bool vec at C level)
profmem::profmem(coalesce(x, 0))
#> Rprofmem memory profiling of:
#> coalesce(x, 0)
#> 
#> Memory allocations:
#> Number of 'new page' entries not displayed: 3
#>        what     bytes                                                            calls
#> 3     alloc       216                                 coalesce() -> vec_ptype_common()
#> 4     alloc       216                                  coalesce() -> vec_size_common()
#> 6     alloc  40000048 coalesce() -> map() -> lapply() -> FUN() -> vec_detect_missing()
#> 7     alloc  40000048                         coalesce() -> map() -> lapply() -> FUN()
#> 8     alloc  80000048                                        coalesce() -> <Anonymous>
#> 9     alloc  10000048                                        coalesce() -> <Anonymous>
#> total       170000624