Replace dpx_pdfobj.rs with lopdf crate

[cormacrelf]

It's a perfectly matched replacement for that 4.8kLOC file.

https://lib.rs/crates/lopdf

There's also https://github.com/fschutt/printpdf (which uses lopdf), which does a lot of what dpx does already, but I'm guessing it'll be a bit difficult to integrate. Might be a target to work towards -- maybe emulating printpdf's approach to some things would help craft PRs against printpdf that would eventually bring it up to feature parity with dpx's PDF writing support.

printpdf has a pretty good wiki on how PDF documents are created, may be useful.

You can also probably replace dpx_pdfparse.rs and a few other things.

[XVilka]

Sounds like a good idea!