SP1PublicValues handling for the aggregation proof

[shriphani]

The aggregation proof serializes the buffer of an SP1PublicValues as a Vec<u8> and sends it over to the guest. If the guest wants to read the actual program output, it needs to use bincode::read on this value. Ideally the user shouldn't have to explicitly reference both values. Can we expose SP1PublicValues or some other object where we can do the following:

Host:

let proof1 = ...; // an existing proof

// write proof1 public values to guest
aggregation_guest_stdin.write(proof1.public_values);
// also write the vk value

Guest:

let proof: SP1PublicValues = sp1_zkvm::io::read();
sp1_zkvm::precompiles::verify::verify_sp1_proof(program_identifier, proof.digest());
let program_output: OutputStruct = proof.output();

basically, pub use SP1PublicValues so it isn't hidden in sp1_core, and implement a digest and output method for it so guests don't have to clone bincode.