Add efficient compression for test images#4

The test images for FAT8 floppies are ~250KiB each. Other file systems will have much larger images. To avoid having users download a gigabyte of test data, we should compress these. Some quick experimentation shows that run-length encoding the image^* and then gzipping the result gives great results, so we're implementing that here. For reference, by "great results" I mean that an empty FAT8 floppy image can be compressed to 68 bytes. We'll see how it works with others. _{*RLE8, as used by Microsoft's BMP image format.}