Faster version of arg_match()

[krlmlr]

because it's very slow. Example: more than half of the overhead in vctrs::vec_as_location2() comes from arg_match().

Downstream: tidyverse/tibble#780.

vctrs dev

library(vctrs)
library(magrittr)

bench::mark(
  vec_as_location2("x", 1L, "x", missing = "propagate", arg = "x"),
  iterations = 1000
) %>%
  dplyr::select(-expression)
#> # A tibble: 1 x 5
#>        min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1   78.7µs   88.8µs     9451.     322KB     18.9

^{Created on 2020-06-14 by the reprex package (v0.3.0)}

r-lib/vctrs#1145

library(vctrs)
library(magrittr)

bench::mark(
  vec_as_location2("x", 1L, "x", missing = "propagate", arg = "x"),
  iterations = 1000
) %>%
  dplyr::select(-expression)
#> # A tibble: 1 x 5
#>        min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1   25.7µs   34.9µs    26573.     172KB     26.6

^{Created on 2020-06-14 by the reprex package (v0.3.0)}