Truncate sha-256 instead of splitting output into two Fr fields

[jeanmon]

• Let the new design be vetted by another pair of eyes (e.g.: Patrick)
• Let us consider whether Keccak or Blake is a potential alternative. (We do need the hash function to be 'Ethereum compatible' though)

[dan-aztec]

is it OK to use the stdlib hash_to_field, which is based on blake2s, or do we want to keep sha256?

Will start with just truncating sha256. I see also that calldata hash and L1toL2Messages hash can also be reduced to a single Field element in addition to Encrypted/Unencrypted Logs hash