bug: Change U128 serde to respect "canonical" Noir serde

[Thunkar]

It generates a myriad of problems and difficult to debug issues. U128 currently is a struct with two limbs, well, let's (de)serialize it as such and not "collapse" it into a Field. Further reading: https://hackmd.io/@aztec-network/Hkbo4p9gR?type=view

Note: Because of this issue we have a lot of conversions to and from U128 in our codebase (e.g. Token::balance_of_public converts U128 to a Field before returning the value). Once this is tackled find all these conversions and remove them. Look for TODO(#8271) in the codebase.

[benesjan]

Relevant discussion

[nventuro]

Related to #7728