* install: implement `-C` / `--compare`
GNU coreutils [1] checks the following: whether
- either file is nonexistent,
- there's a sticky bit or set[ug]id bit in play,
- either file isn't a regular file,
- the sizes of both files mismatch,
- the destination file's owner differs from intended, or
- the contents of both files mismatch.
[1] https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/install.c?h=v8.32#n174
* Add test: non-regular files
* Forgot a #[test]
* Give up on non-regular file test
* `cargo fmt` install.rs
- refactor internal version specifications to be ">=M.m.p" (where M.m.p is *already published*)
## [why]
Loosening internal version dependencies decreases the coupling between packages such
that packages can be published in a looser order. It allows the packages to be version
updated and published in tandem (ie, by using `cargo workspace ...`). Once published,
the internal versions can then be updated (again, to an *already published* package
version), as needed.
