Instead of having possibly multiple FatFS objects, each with its own
`fs` struct, there is one global static fs_instance. This is to match
the mode of operation of ff.c, which assumes a global list of mounts,
and all functions operate on the global based on path.
Methods of FatFS were converted to functions on the fatfs module.
fatfs.unmount() does not call ff.c's unmount, but simply invalidates
fs_instance. This is basically what ff.c would do, except without
messing with ff.c's global list of mounts.
