Put data in merkle tree in epoch tree leaves

[vimwitch]

Right now we store user data as a hashchain in the epoch tree leaf. Instead we can store the data in a merkle tree, and put the root in the epoch tree leaf. This will reduce the gas cost of attestations. We could call this the data tree.

In the data tree the first leaf should be the epoch key, then the following leaves are the data fields.

For a config of FIELD_COUNT=6 we'll use 3 hashes per attestation (onchain) instead of 5. The number of hashes in ZK stays the same.