Method generic_instance_bases

The base idents (`generic_ident`) of every registered generic instance, e.g. `Foo` for the instance `Foo `. For a generic EXTERN base this is the only place the bare base name lives: a generic extern (`Foo = _CDDL_CODEGEN_EXTERN_TYPE_`) is registered as a plain `Extern` rust struct, but the wasm crate never names it — wasm-bindgen can't express generics, so the instance collapses to the arg

(&self)