OA: cd with ~ and symlinks

Write cd(current_dir, destination, home_dir=None, symlinks=None): the directory you'd be in after running cd destination from current_dir and then pwd. It returns a string. You'll need home_dir from level 2 and symlinks from level 3. Ignore them for now.

In this level destination is always relative, and current_dir is an absolute, already normalized path such as /srv/app.

• Split on /. Empty pieces (from //, or a leading or trailing slash) and . do nothing.
• .. moves up one directory. At / it stays at / and doesn't raise an error.
• Anything else is a directory name to step into.
• The result is always absolute and normalized. It starts with /, has no ., .. or empty pieces, and has no trailing slash. The root is exactly "/", never "".

cd("/srv/app", "logs")             # "/srv/app/logs"
cd("/srv/app", "../lib//x/")       # "/srv/lib/x"
cd("/srv/app", "./a/b/../")        # "/srv/app/a"
cd("/srv", "../../../..")          # "/"
cd("/", "..")                      # "/"

Don't touch the real filesystem. This is pure path processing.