Build the new (yet unused) style system as part of libscript, to avoid bitrot.

I started this in a [separate repository](https://github.com/SimonSapin/servo-style), and imported it into `servo/src/components/script/style` after some Rust minor upgrades.

I move this into the script crate because it’s gonna both need stuff there (the content tree, for selector matching) and be needed by stuff there (the HTML parser calls the CSS parser). As far as I know, we can not have circular dependencies between crates.