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

[clarfonthey]

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.
• 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.