[feature request] Share disk space between different dists (for the same releases)

[avodonosov]

If two dists contain many equal releases, the disk space is wasted by storing duplicated files, because every dist downloads tarballs and stores unpacked sources in its own private directory.

This may be solved by using shared FILE-STORAGE directory for all dists, if we can guarantee unique naming. Good candidate for unique name is the tarball hash sum, especially because releases.txt already contains hash sums. (http://en.wikipedia.org/wiki/Nix_package_manager uses similar solution to store packages on file system, although in their case hash is not just source code hash, but includes also build options)

Disadvantage of hash-named directories - complicates file system navigation for quicklisp hacker, because instead of readable names like alexandria-20130128-git/ he will deal with names like 2dc75635c4d6a641618a63e61a9aa98252bbd2b7.

May be solved by prepending library name to the hash sum: alexandria-20130128-git-2dc75635c4d6a641618a63e61a9aa98252bbd2b7/ . But beware, Windows has file path limit - 260 without using special notation. So, maybe just alexandria-2dc75635c4d6a641618a63e61a9aa98252bbd2b7