WISH: Deploy futurize transpilator on package load

[HenrikBengtsson]

Add a function futurize_apply_to_package() that can be used in a package's .onLoad() to apply all futurize() transpilations on package load, e.g.

.onLoad <- function(libname, pkgname) {
  futurize::futurize_apply_to_package()
}

Special care has to be take for arguments such as when and eval.

Benefits

• It removes the tiny overhead of the futurize() transpiler that otherwise applies at each call.
• It validates that all futurize() transpilations are valid up front.
• ...?