cpp implementation of URLdecode

[schochastics]

ref #13

urls <- rep("https://www.google.co.jp/search?q=\u30c9\u30a4\u30c4",10000)
bench::mark(
    cpp = url_decode(urls),
    URLdecode = URLdecode(urls),
    iterations = 1, check=FALSE
)
# A tibble: 2 × 13
  expression      min   median `itr/sec` mem_alloc `gc/sec` n_itr  n_gc total_time result
  <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl> <int> <dbl>   <bch:tm> <list>
1 cpp          10.3ms   10.3ms     96.9     80.7KB      0       1     0     10.3ms <NULL>
2 URLdecode   390.6ms  390.6ms      2.56    78.2KB     12.8     1     5    390.6ms <NULL>
# ℹ 3 more variables: memory <list>, time <list>, gc <list>

[chainsawriot]

It would better to change this to unsigned int

[chainsawriot]

The little benchmark in #22 is now 82ms; solid improvement from >1000ms.