Replace AztecU128 with the U128 struct in Noir's stdlib

[kevaundray]

## Problem

These two structs serve roughly the same purpose. The Aztec version was made just for type safety reasons, so it doesn't apply the necessary constraints.

## Solution

Replace usage of Aztec's version of U128 with Noir's. This will likely reduce performance by some delta given that Noirs version uses two u64s and Aztec's version uses a Field.

We can make the transition smoother by adding from_field to Noir's version of U128