md5sum and recursive traversal of dirs

[Сергей Кузнецов]

Hello, I find it strange that md5sum does not yet support recursive
directory traversal. I moved part of the code from ls and added this
functionality. How about adding this? I also added sha3 and md6 algorithms,
they are in "gl/lib/".
By the way, I wrote two new small programs: xchg (or swap, which name is
better?) And exst (exit status). The first program simply changes files,
the number of which can be more than two. The second program launches the
program indicated at startup and, after its completion, prints the output
status or the caught signal.
I don't know how to commit my code in yours repo (is it even possible?), so
I write this letter.

Hope to feedback from you soon,

Sergey