Log Message:
(devel/R-xfun)  Updated 0.17 to 0.18

# CHANGES IN xfun VERSION 0.18

## NEW FEATURES

- Added a function `grep_sub()` to perform replacement with `gsub()` on elements
  matched from `grep()`.

- Added a function `github_releases()` to obtain the tags from the Github
  releases of a repo.

- Added a function `bump_version()` to increase the last digit of version
  numbers by one.

- Moved a function `process_file()` from the **blogdown** package to this
  package, and documented it.

- Added a function `valid_syntax()` to check if an R code fragment is
 syntactically valid. This function was moved from the **highr** package.

- Added a function `url_filename()` to extract filenames from URLs.
  This function is used by `download_file()` to determine the default
  output filename.

- Added a function `do_once()` to perform a task once in an R session.

- Added a function `proj_root()` to find the root directory of a project.
  Currently it only supports R package projects and RStudio projects by default.

- Added a function `relative_path()` to calculate the relative path of a path
  relative to a directory.

- Added a function `from_root()`, which is similar to `here::here()` but
  returns a relative path instead of an absolute path.

- Added a function `magic_path()` that, given an incomplete input path,
  tries to find the actual path recursively under subdirectories of a root
  directory. For example, users may only provide a base filename, and
  `magic_path()` will look for this file under subdirectories and return
  the actual path if it is found.

## MINOR CHANGES

- Now `download_file()` tries the download method `winnet` first (previously
  it was `libcurl`) on Windows (thanks, @cderv, #33).