This makes debugging easier, you can pass an AUMHash to a printf and get
a string that is easy to debug.
Also rearrange how directories/files work in the FS store: use the first
two characters of the string representation as the prefix directory, and
use the entire AUMHash string as the file name. This is again to aid
debugging: you can `ls` a directory and line up what prints out easily
with what you get from a printf in debug code.
Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
15b8665787