On Windows, when you create a symbolic link you must specify whether it
points to a directory or a file, even if it is created dangling, while
on Unix, the same symbolic link could point to a directory, a file, or
nothing at all. Furthermore, on Windows special privilege is necessary
to use a symbolic link, while on Unix, you can generally create a
symbolic link in any directory you have write privileges to.
This means that it is unlikely to be able to use symbolic links purely
portably; anyone who uses them will need to think about the cross
platform implications. This means that using platform-specific APIs
will make it easier to see where code will need to differ between the
platforms, rather than trying to provide some kind of compatibility
wrapper.
Furthermore, `soft_link` has no precedence in any other API, so to avoid
confusion, move back to the more standard `symlink` terminology.
Create a `std::os::unix::symlink` for the Unix version that is
destination type agnostic, as well as `std::os::windows::{symlink_file,
symlink_dir}` for Windows.
Because this is a stable API, leave a compatibility wrapper in
`std::fs::soft_link`, which calls `symlink` on Unix and `symlink_file`
on Windows, preserving the existing behavior of `soft_link`.
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