Rewrite cdup.zsh using a much better design.

Previously, cdup relied on an external script, but still didn't work
that well. This version relies on readlink to detect if we've reached
the root directory, which is much, much cleaner than having to cd all
the way up the tree just to be able to test for root.
Modify and add development aliases.
Fix an issue with $HOME not being collapsed to ~ in the current path.

Commit 3660997eff3eaa91fecfad30fc67cc52939210ce inadvertantly broke ~
replacement in the path of non VCS directories. This fixes that.