Use std::intrinsics::TypeId

Fix indentation + update comment
auto merge of #3539 : jamesberlage/servo/fix/meaningful-is, r=jdm

This is intended to address #3488.  [AnyPrivate](http://doc.rust-lang.org/core/any/trait.AnyPrivate.html) is now in the Rust core, so we can leverage that instead of writing an implementation.