Use `Any` for the layout data.

Breaks the dependency between `gfx` and `script`, which is nice.
auto merge of #1208 : pcwalton/servo/any-layout-data, r=metajack

Breaks the dependency between `gfx` and `script`, which is nice.

This exposed some performance issues with Rust's `Any` type, which I've filed:

https://github.com/mozilla/rust/issues/10382