Release cpp11 0.5.4

Prepare for release: * [x] `git pull` * [x] Check [current CRAN check results](https://cran.rstudio.org/web/checks/check_results_cpp11.html) * [x] [Polish NEWS](https://style.tidyverse.org/news.html#news-release) * [x] `urlchecker::url_check()` * [x] `devtools::check(remote = TRUE, manual = TRUE)` * [x] `devtools::check_win_devel()` * [x] Update `cran-comments.md` * [x] `git push` Submit to CRAN: * [x] `usethis::use_version('patch')` * [x] `Sys.setenv("CPP11_EVAL" = "true"); devtools::submit_cran()` * [x] Approve email Wait for CRAN... * [x] Accepted :tada: * [x] `usethis::use_github_release()` * [ ] `usethis::use_dev_version(push = TRUE)`