Consider how to expose reference types

[Typed function references](https://github.com/WebAssembly/function-references/blob/b158867a0cb5c24a31ee9a31c2169300bfae7586/proposals/function-references/Overview.md#type-representation) proposes exposing reference types as `{ref: ConsType, nullable: bool}`. Should this pattern also be used for funcref/externref?