util: add a version comparison class based on rpm version rules

This (strives to) implement the same algorithm that rpm uses [1].
All strings are accepted, and the answer is always -1, 0, or +1.
This should be also compatible with Debian rules: Debian doesn't
allow ^ in versions, but otherwise follows similar rules. The addition
of ~ in rpm was modelled after Debian. If Debian adds support for ^
in the future, hopefully it'll be in a way that is compatible.
(rpm and deb have different rules how to split the package name
version parts, but this shouldn't matter for us, since we're only
looking at the version part here.)

For all not overly complicated strings this should generally return the
result that people expect. For some of the corner cases the results
might be nonobvious, but I think it's better to follow one established
standard than to try to come up with something fresh.

[1] So… I should put a link to some docs here. But I don't think
    the are is any reference for this. Oh, rpm :---[
    https://github.com/rpm-software-management/rpm/blob/master/rpmio/rpmvercmp.c#L12

[2] https://www.debian.org/doc/debian-policy/ch-controlfields.html#version

Note on the copyright: I wrote this from scratch based on my
understanding of the rules and examples online and the (scant)
documentation that is online. The example list is copied from
rpm tests, but the way that the examples are structured and invoked
is completely different. The only part that was carried over are
the version strings to test, so I think it's reasonable to treat
this as an indepedent work.