Start to bring cksum up to date with hashsum

[tertsdiepraam]

For context see: #4414

cksum is lagging behind hashsum and needs to be updated. One way to do with would be to share functions between the two, but I didn't want to do that, because I do not like many aspects of the current hashsum implementation. So instead I want cksum to be the base of all the cksum-like utilities. But because that's a lot of work I'll take some slow steps first.

It's a draft because I need to add some tests.

[sylvestre]

@tertsdiepraam is it ready ?

[sylvestre]

a few clippy warnings:

error: constants have by default a `'static` lifetime
  --> src/uu/cksum/src/cksum.rs:54:24
   |
54 |     pub const BINARY: &'static str = "binary";
   |                       -^^^^^^^---- help: consider removing `'static`: `&str`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_static_lifetimes
   = note: `-D clippy::redundant-static-lifetimes` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::redundant_static_lifetimes)]`

[sylvestre]

no activity for a while, closing