Add check_linked_version()

[lionel-]

Implements check_linked_version() in a standalone file that can be copied in zero-dep packages. After some testing in other packages it will be exported from rlang as well.

Packages that include native code are fragile on Windows and might not be properly updated. When the package is loaded in memory, R might install a new version of the package without updating the corresponding DLL because. With check_linked_version() you can preventively inform your users about broken installation.

It should be called from the .onLoad() hook:

.onLoad <- function(lib, pkg) {
  check_linked_version(pkg, with_rlang = FALSE)
}

It expects that the namespace of the checked package contains a C callable called *pkgname*_linked_version(). This callable should return the package version as a string. The version recorded in the native library is then compared to the one returned from packageVersion(). If they don't match, an informative error is thrown:

Error: The vctrs package is not properly installed.
✖ The DLL version (0.10.99.9005) does not correspond to the package version (0.2.99.9005).
ℹ Please update vctrs with `install.packages("vctrs")` and restart R.

On Windows the error message includes additional advice from rstats-wtf/what-they-forgot#62 about safely reinstalling packages:

Error: The vctrs package is not properly installed.
✖ The DLL version (0.10.99.9005) does not correspond to the package version (0.2.99.9005).
ℹ Please update vctrs to the latest version.
ℹ Updating packages on Windows requires precautions:
  <https://github.com/jennybc/what-they-forgot/issues/62>

A usethis tool will eventually make it easy to generate an up-to-date version.c file at each version bump.