parse_aurinfo: add checksums as multivalued fields
parse_aurinfo -> aurinfo
disallow overriding pkgver/pkgrel/epoch in packages

This follows a recent change in pacman, 8a02abcf194.
create a testutil and some unit tests

This already found me 2 bugs...
refactor README

this is getting out of date, so cut back the scope, and remove the
mention of "proof of concept". This battlestation is more or less fully
operational.