[bugadani]

• Dir::rename writes the new entry before it frees the old one. A full destination no longer loses the file.
• Short name generation does not panic on a name that starts with a non-ASCII character, or on an empty name.
• A full FAT12/16 root directory returns NotEnoughSpace, and a write at the 4 GiB file limit returns WriteZero. Before, write returned Ok(0), which made write_all panic.
• BufStream flushes a dirty cached block before an aligned read that covers it, and discards a cached block that an aligned write replaces.

Fixes #71