a * b = 2**128 * c + d.
The guarantee is verified by u128_mul_guarantee_verify, which is the only way to destruct this
type. This way, one can trust that the guarantee holds although it has not yet been verified.
core::integer
core::integer::U128MulGuarantee
A type that contains 4 u128s (a, b, c, d) and guarantees that