dst may or may not exist. In case it exists it might already be a symlink.
In neither case we should try to canonicalize dst, only its parent directory.
https://www.gnu.org/software/coreutils/manual/html_node/ln-invocation.html
> Relative symbolic links are generated based on their canonicalized
> **containing directory**, and canonicalized targets.
* bump the minimal version of rustc to 1.32
* feature(ln): implement -r
* fix two issues
* Use cow
* rustfmt the change
* with cargo.lock 1.31
* try to unbreak windows