Rework case_when() on top of vctrs::vec_case_when()

[DavisVaughan]

Finally able to remove dplyr:::vec_case_when() 🎉 , it has served us well over the years 🫡

---

100 million values, 3 conditions

cross::bench_branches(\(x) {
  library(dplyr)
  set.seed(123)

  values <- list(
    sample(1000000, 1e8, replace = TRUE),
    sample(1000000, 1e8, replace = TRUE),
    sample(1000000, 1e8, replace = TRUE)
  )

  conditions <- list(
    sample(c(TRUE, FALSE), 1e8, replace = TRUE),
    sample(c(TRUE, FALSE), 1e8, replace = TRUE),
    sample(c(TRUE, FALSE), 1e8, replace = TRUE)
  )

  bench::mark(
    case_when(
      conditions[[1]] ~ values[[1]],
      conditions[[2]] ~ values[[2]],
      conditions[[3]] ~ values[[3]]
    ),
    iterations = 5
  )
})

# A tibble: 2 × 14
  branch        expression      min  median `itr/sec` mem_alloc `gc/sec` n_itr  n_gc total_time result
  <chr>         <bch:expr> <bch:tm> <bch:t>     <dbl> <bch:byt>    <dbl> <int> <dbl>   <bch:tm> <list>
1 feature/case… case_when… 158.15ms 173.4ms     5.77    381.6MB     8.65     2     3    346.8ms <int> 
2 main          case_when…    2.23s   2.25s     0.428     4.1GB     1.11     5    13      11.7s <int> 
# ℹ 3 more variables: memory <list>, time <list>, gc <list>

The most satisfying part is looking at the memory allocations. This is the result of A LOT of vctrs work! 🎉

With this PR:

Memory allocations:
Number of 'new page' entries not displayed: 1
       what     bytes                                                                    calls
1     alloc       280 case_when() -> case_formula_evaluate() -> new_environment() -> new.env()
2     alloc       216              case_when() -> case_when_size_common() -> vec_size_common()
3     alloc       216              case_when() -> case_when_size_common() -> vec_size_common()
5     alloc       216                                           case_when() -> vec_case_when()
6     alloc 400000048                                           case_when() -> vec_case_when()
total       400000976   

With CRAN dplyr:

Memory allocations:
       what      bytes                                                                                            calls
1     alloc        280                         case_when() -> case_formula_evaluate() -> new_environment() -> new.env()
2     alloc        216                                                                 case_when() -> vec_size_common()
3     alloc        216 case_when() -> vec_case_when() -> names_as_error_names() -> vec_paste0() -> vec_recycle_common()
4     alloc        216 case_when() -> vec_case_when() -> names_as_error_names() -> vec_paste0() -> vec_recycle_common()
5     alloc        216                                             case_when() -> vec_case_when() -> vec_ptype_common()
6     alloc  400000048                                                      case_when() -> vec_case_when() -> vec_rep()
7     alloc  400000048                                                                   case_when() -> vec_case_when()
8     alloc  400000056                                                        case_when() -> vec_case_when() -> which()
9     alloc  199973472                                                        case_when() -> vec_case_when() -> which()
10    alloc  400000048                                                                   case_when() -> vec_case_when()
11    alloc  400000056                                                        case_when() -> vec_case_when() -> which()
12    alloc  100008272                                                        case_when() -> vec_case_when() -> which()
13    alloc  400000048                                                                   case_when() -> vec_case_when()
14    alloc  400000056                                                        case_when() -> vec_case_when() -> which()
15    alloc   50016976                                                        case_when() -> vec_case_when() -> which()
16    alloc  400000056                                                        case_when() -> vec_case_when() -> which()
17    alloc   50001472                                                        case_when() -> vec_case_when() -> which()
18    alloc  199973472                                                    case_when() -> vec_case_when() -> vec_slice()
19    alloc  100008272                                                    case_when() -> vec_case_when() -> vec_slice()
20    alloc   50016976                                                    case_when() -> vec_case_when() -> vec_slice()
21    alloc   50001472                                                  case_when() -> vec_case_when() -> vec_recycle()
22    alloc  400000048                                                  case_when() -> vec_case_when() -> list_unchop()
total       4400001992