(devel/R-xfun) Updated 0.19 to 0.20

# CHANGES IN xfun VERSION 0.20

## NEW FEATURES

- Added a function `msg_cat()` to generate a message with `cat()`. See
  the help page `?xfun::msg_cat` for more information.

- Added a function `mark_dirs()` to mark directories with a trailing
  slash in a vector of paths to differentiate them from normal
  filenames (#44).

## BUG FIXES

- `xfun::proc_kill()` failed to work on *nix.

- `xfun::del_empty_dir()` failed to delete empty dirs.

- `xfun::file_string()` preserves emptiness (thanks, @MichaelChirico,
  #38).

- `xfun::raw_string()` preserves the class(es) of the input now
  (thanks, @MichaelChirico, #40).

## MINOR CHANGES

- Exported the function `dir_create()`.

(mef)