fix(AggLayer): use correct byte<>Felt conversion

[mmagician]

Switch to using bytes_to_packed_u32_felts which reads the bytes as LE instead of BE, eliminating the mismatch:

// Compute expected GER using keccak256
let expected_ger_preimage = KeccakPreimage::new(ger_preimage.clone());
let expected_ger_felts: [Felt; 8] = expected_ger_preimage.digest().as_ref().try_into().unwrap();

let ger_bytes = felts_to_u256_bytes(expected_ger_felts);

let ger = ExitRoot::from(ger_bytes);
// sanity check
assert_eq!(ger.to_elements(), expected_ger_felts); // <----------- this would fail, bc. to_elements called into BE-conversion

while at it, I unified the underlying representation of ExitRoot and SmtNode to a single Keccak256Output (any thoughts on the naming?) and use type aliases instead.

[partylikeits1983]

Looks good!

However, should we merge agglayer-fixed-2 into next, then delete agglayer and create a new agglayer branch which is up to date with next?

Then rebase this PR off of next?