Add new location for Rust, also add Servo submodules

`mozilla/rust` was moved to [`rust-lang/rust`](github.com/rust-lang/rust) (along with the Rust playpen and related repos).


Additionally, a large amount of Servo code is in submodules kept in the mozilla-servo organization. Not all of these are maintained, by Mozilla, though (eg rust-http, rust-encoding — I haven't included these in the PR for that reason; they have upstreams and we just pull changes)