Prevent incomplete Eq implementations from allowing unintentional mutability when using hints

[TomAFrench]

See: #5940 (comment)

It's a common pattern for developers to perform logic in a unconstrained function and then assert that the output is correct (due to this being cheaper than calculating the result in a constrained context). This relies on the implementation of Eq used by the types crossing the unconstrained boundary asserting an equality on every field within that type.

In the case where the Eq implementation doesn't cover all fields within the relevant type then the prover will be able to mutate any fields which are not asserted on.

We should consider whether we can fix this with a ConstrainEq trait to be used in these situations. However this could result in confusion aroundassert_eq and other functions.