derive debug trait for FhewBoolEvaluator

[ChihChengLiang]

This PR adds the Debug trait to FhewBoolEvaluator

However, I'm not sure if I'm doing the right way.

The need for the debug trait is reasoned as follows:

1. The server has to deserialize encrypted bit ciphertext for the computation.
2. We wrap the deserialized ciphertext with FheBool, because it has the BitAnd, BitOr, and BitXor implemented.
3. So my deserialization method have the output signature Vec<FheBool<FhewBoolEvaluator<R, M>>>
4. This comes with a problem if I want to unwrap the vector to fit a slice. Because unwrap requires a Debug trait for FhewBoolEvaluator

https://github.com/ChihChengLiang/haunted/pull/5/files

[ChihChengLiang]

Debug is already added in #41

[han0110]

Ideally clients will only produce FhewBoolCiphertextOwned<R::Elem> and serialize it to send to server, and server will deserialize then wrap it with evaluator by FheBool::from_ct(evaluator, ct) or FheU8::from_cts(evaluator, cts) for further computation. Then we can avoid client side to handle evaluator stuff.