Derive `TyEncodable`/`TyDecodable` in `rustc_type_ir` when `derive(TyEncodable)` or `derive(TyDecodable)` sees an `I` type parameter on a struct that has no `'tcx`, then parameterize the `TyEncoder`/`TyDecoder`'s interner over that variable rather than `TyCtxt<'tcx>`. Also, emit where clauses for fields rather than generics.