RFC: Remove #[crate_id]

Your updated version actually doesn't address any of my concerns here. And it is metadata, but the only thing the metadata is used for is as input to the hash. But presumably it's also stored in the crate and can be extracted when reading the crate data, right? So it can be used by tools that operate on crates to provide information (e.g. cargo could theoretically use it to record the source the crate was installed from, and then read it later to validate that the source matches the crate).

Also if my suggestion to allow for specifying metadata attributes on the `extern crate` line is implemented, then this metadata would obviously be used for that as well.

I don't feel like a `metadata(...)` section implies that's the full extent of the data that leads to the hash; after all, the entire section is optional, and crates with no `metadata(...)` section won't have the same hash.