Consider custom-built `libarchive` bindings instead of `libarchive3` crate

So, state of things right now: * `libarchive3` is a very old, unmaintained crate with many issues. * `libarchive2` exists as a newer crate, [but it's slop](https://github.com/AllenDang/libarchive-rs/issues/12). * There don't appear to be any good alternatives. Considering how little of libarchive is needed to make this work, it feels like the best option is to include the necessary libarchive bindings directly in the `alpm-sys` crate and use those instead. Or, if someone maintaining this crate wants to go above and beyond and make a better libarchive binding crate, feel free, but that seems like way more work than necessary to make the mtree support work.