This brings a massive speedup: 58.768s -> 2.010s on my sbuild disk.
We spend most of time zeroing a buffer to read files with 1-2 extents.

Noticing this is Zygo Blaxell's fault: praise him with great praise!

Closes: #30

_FILE_OFFSET_BITS is already set to 64

Closes #19

In Debian they got split out of btrfs-progs into libbtrfs-dev.

Even though we're root, can still happen on sshfs or such mounts.

Doubled / is harmless (not on Windows...), but would confuse people.

The latter should be easy to fix if the radix implementation is replaced.

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>

Self-explaining now, so removed.

Closes #12

A non-sparse compressed file without extra fragmentation will hit this
at 24GB.

Closes #10

Closes #11

It wasn't obvious enough.

Closes #8

The table is misaligned for >= 1 drivemaker's terabyte, but as this output
isn't really meant for human reading, this probably doesn't matter.

This disallows loop-local variable declarations.