RFC: Remove #[crate_id]

You said the other attributes are metadata, but you didn't say that attributes not listed here are legal to write. It's a bit confusing, I thought you might have been saying e.g. `name` is also metadata (just metadata that the compiler uses).

I think it's more sensible to nest the metadata into a `metadata` list, otherwise I would expect e.g. `-C name=foo` or `-C type=rlib` to work (and I don't think the former should work, and the latter definitely shouldn't).