Rename Binary to Bytes in the Val and Spec XDR

Binary is a somewhat confusing term. When I see it in the docs it doesn't scream at me that this is the type for storing byte slices or byte arrays. Doing a quick survey of Ethereum, they use the term Bytes in both Solidity and Vyper. Thinking back to whenever I have explained Binary to others or in my head, I immediately reach for the term "bytes" or "byte slice".

The term Bytes is what we're using at the SDK layer now since stellar/rs-soroban-sdk#394.