The dual scraptchpad thing doesn't work. Things become very
complicated when it's time to write that back to the file. We
overwrite our contents and end up with garbage.
This hard-binds ed to the filesystem (I liked the idea of working
only with blockdevs though...), but this is necessary for the
upcoming `w` command. We need some kind of way to tell the
destination to write to truncate itself.
This only has a meaning in the filesystem, but it's necessary to
let the file know that its registered file size has possibly
shrunk.
I thought of alternatives that would have allowed me to keep ed
blkdev-centered, but they were all too hackish to my own taste.
Hence, this new hard-bind on files.
