Filesystem support

[thejpster]

The block device API is going back into the BIOS API, so we can work on filesystem support using the Linux BIOS.

[thejpster]

We can do DIR now in #36. Next step is a LOAD command - and to change RUN to verify that the application loaded is actually valid before jumping to it so you can't jump to garbage.

[thejpster]

Wrote a LOAD command

[thejpster]

We'll need an array of "open file" objects. The file handle you give to a user will be an index into this array. Each object will have a volume index and a file handle for that volume. If a card is ejected, all files on that volume will be marked as errored. All open files will need to be closed when a program exits.