Implement no-op hasher for Id's 

Since both [`DocumentId`](https://github.com/automerge/spanreed/blob/6af302a0b76101ef9fd628c71e19e282a41d3af5/src/interfaces.rs#L22) and [`RepoId`](https://github.com/automerge/spanreed/blob/6af302a0b76101ef9fd628c71e19e282a41d3af5/src/interfaces.rs#L7) are meant to be unique, we could avoid hashing them for use in maps. 