Port rsvg-convert to Rust

Rsvg-convert is the only executable we build; it's a small command-line program still done in C. It should be ported to Rust and made to use the librsvg crate API.

Since a bunch of external tools depend on rsvg-convert, we should make sure that the exact behavior of the command-line arguments is preserved. I think structopt can do this with no problems. Ideally we should have tests for the rsvg-convert binary to ensure that it works in the same way as the C version.