Eliminate `CrateMetadataRef`.

There are a number of things I dislike about `CrateMetadataRef`.
- It contains two fields `cstore` and `cdata`. The latter points to data
  within the former. It's like having an `Elem` type that has a
  reference to a vec element and also a reference to the vec itself.
  Weird.
- The `cdata` field gets a lot of use, and the `Deref` impl just derefs
  that field. The `cstore` field is rarely used.
- `CrateMetadataRef` is not a good name.
- Variables named `cdata` sometimes refer to values of this type and
  sometimes to values of type `CrateMetadata`, which is confusing.

The good news is that `CrateMetadataRef` is not necessary and can be
replaced with `&CrateMetadata`. Why? Everywhere that `CrateMetadataRef`
is used, a `TyCtxt` is also present, and the `CStore` is accessible from
the `TyCtxt` with `CStore::from_tcx`.

So this commit removes `CrateMetadataRef` and replaces all its uses with
`&CrateMetadata`. Notes:
- This requires adding only two uses of `CStore::from_tcx`, which shows
  how rarely the `cstore` field was used.
- `get_crate_data` now matches `get_crate_data_mut` more closely.
- A few variables are renamed for consistency, e.g. `data`/`cmeta` ->
  `cdata`.
- An unnecessary local variable (`local_cdata`) in `decode_expn_id` is
  removed.
- All the `CrateMetadataRef` methods become `CrateMetadata` methods, and
  their receiver changes from `self` to `&self`.
- `RawDefId::decode_from_cdata` is inlined and removed, because it has a
  single call site.