Remove fs from the parameters of Decider::preprocess

[winderica]

In the current Decider trait, the preprocess method takes fs, an instance of the FoldingScheme trait, as input.

However, note that fs is created by the prover by running FoldingScheme::init with the initial state z_0. On the other hand, Decider::preprocess is usually invoked by a trusted party at the beginning (before provers and verifiers are involved), when z_0 is still unknown.

Thus, it makes more sense to remove the fs parameter from Decider::preprocess. The implementations of Decider are updated correspondingly, so that the preprocessing can be done without a concrete fs. Instead, we simply build a dummy decider circuit, during which we only need to provide some parameters that are known before the creation of fs.

[arnaucube]

Nice! More clean usage of the Decider now!