Symlink support?

This is platform specific as it only is for unix, but is there any consideration on supporting symlinks for child paths where you have something like

```rust
let temp = assert_fs::TempDir::new().unwrap();

// Make a real file
let file = temp.child("file");
file.touch().unwrap();

// Make a symlink to a file (only available on unix platform)
temp.child("link").link(file).unwrap();
```