Introduce proto definitions for `Word`, `StorageMapKey`, etc.

Hmm I had a comment about this somewhere, but I think it got lost in the process:
Yes, I would be in favor of introducing different proto definitions for `Word`, `StorageMapKey` etc., even if under the hood they hold the same data structure. But since we're already using `Digest` when we really mean just `Word`, this would be a rather broader refactor to make things consistent across the board.

_Originally posted by @mmagician in https://github.com/0xMiden/node/pull/1743#discussion_r2882783262_