Initial ext4 support

[arihant2math]

Redone version of #61. Uses a patched version of ext4-view which has been adapted to be async.

Advantages over fat32:

• soft/hard link support
• fast directory traversal
• larger files sizes

To test the changes:

# Move old image
mv moss.img fat_img.img
# Populate ext4_mount/ with the need stuff (just copy over from the fat mount) 
touch moss.img
mkfs.ext4 moss.img 256M
mkfs.ext4 -d ext4_mount moss.img

Then update qemu-runner.sh to change --rootfs=fat32fs to --rootfs=ext4fs`.

After this cargo run -r should work.

Fixes (partially): #4

[arihant2math]

Finally, some progress. ls works!

[arihant2math]

Seems like ls doesn't work, but cat seems to be working.
readdir seems to be the only barrier here.

[hexagonal-sun]

LGTM!