Now that we use `JSTraceable` (defined in `script`), we can create arbitrary implementations on non-`script` types (eg `Url` or `RequestHeaderCollection`) where in the past we had to rely on `Traceable` and `Untraceable` to achieve cross-crate impls of `Encodable`.
This removes the two completely. They can be reintroduced if required, though the `untraceable!` macro should suffice.
Fixes#3469
Source-Repo: https://github.com/servo/servo
Source-Revision: b34df7c343579f200d2e67e21fc566842a4e4a91
The biggest language change is that enum variants now also reserve (for future use) a name in the type namespace, which must not collide with other types. Some things were renamed, and others qualified as `module::name`.
Source-Repo: https://github.com/servo/servo
Source-Revision: 7409685589c550ee7a9f94182f511acddab4c6fd
