Performance: encode_native() does not need an intermediate character vector

[zeehio]

This pull request is applied on top of #36 . Please review that first.

We avoid the intermediate character vector when encoding to native. This makes encode_native() much faster.

This is part of tidyverse/ggplot2#4989, aiming to improve the performance of rasterizing large matrices.

library(farver)
x <- runif(1E7)
ramp <- scales::colour_ramp(c("red", "blue"))
colours_chr <- ramp(x)
spectrum <- farver::decode_colour(colours_chr)

bench::mark(
  before = {
    # This was how encode_native worked before
    cols <- encode_colour(spectrum, alpha = 0.75)
    farver:::encode_native(cols)
  },
  after = {
    farver::encode_native(spectrum,alpha = 0.75)
  }
)
#> # A tibble: 2 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 before        1.16s    1.16s     0.866   114.5MB     0   
#> 2 after       83.53ms  86.04ms    11.7      38.1MB     2.34

^{Created on 2022-09-18 by the reprex package (v2.0.1)}