Remove SecureMul from trait bounds

[danielmasny]

#1021 does not implement SecureMul for any context anymore.

Therefore SecureMul had to be in the trait bounds of several functions.

Task: Remove the trait bound.

Add the trait bound C:Upgradable. Implement SecureMul for all Boolean types for ::DZKPUpgraded. Further, implement SecureMul for all primefield types for ::Upgraded. This would require to change all functions to use the new maliciousAPI and upgrade the context whenever multiplications are used. This should allow to remove SecureMul from the trait bounds.

[danielmasny]

@andyleiserson pointed out that this will not work. Even if we implement SecureMul for Semi Honest DZKPUpgraded and Malicious DZKPUpgraded, which are all structs that implement the DZKPUpgraded trait, the compiler will not know that structs that implement the DZKPUpgraded trait will also have the SecureMul trait.

This could potentially be resolved by a new trait that bundles the SecureMul trait and DZKPUpgraded.