[gecxqaxg] File concatenation in constant time

Create a filesystem that can do (approximately) "cat foo bar > baz" in constant time, by being clever with pointers to data in the background.  Use "copy on write" so baz does not change even if foo or bar does.

Next, do (take bytes M..N of oldfile > newfile) also in constant time.

One could get an interesting web of pointers in the background.

Next, let some segments of a file be "computed", a string of nulls (/dev/zero) is the simplest case.  The next is a random-access compressed data object (though random-accesses compression is not popularly deployed).

Motivation was: pad a file with zeros, loop mount with losetup.