ext2 fs support

I'm writing a module for los0 that aims to support the ext2 fs. So far it can only read regular files (i.e. no symlinks and no writing at all) and is quite slow, but it seems to work (I've tested it only on a bochs disk image formatted with an old version of ext2, but there should be no problems with newer versions). I've written also a sort of common file system interface so that different filesystems can be supported trasparently but it's actually quite a random thing, a real os would need something way better!

I don't expect adding writing support to be too easy because things change a bit from version to version, plus the docs I've found aren't that clear... We'll see. For now, I hope that what I've written can be useful.

Files attached:
partitions.lisp -- reading partition tables (no support for extended partitions)
fs.lisp           -- common fs interface
ext2fs.lisp     -- obvious... :)