Move variables to rust

This is an attempt at fixing #627 (closed).

Moving the integer variables to Rust seems easy enough, however I am struggling with the rsvg_version string. It might be possible to find a solution here, but at the moment I am rather tempted to argue that we remove that constant entirely. As we have only just added it to the ABI a few days ago we can safely remove it. The integer constants should actually be good enough and I do not see a compelling reason to have the rsvg_version string at all. If it's really needed, then let's have a function rsvg_version() that returns the string, preferably heap-allocated