Let `query_position_to_coset` return `gen` and `offset` instead of coset elements

[tsunrise]

Problem Definition

Currently, domain.query_position_to_coset returns all elements in coset. We should make it simply return updated gen and offset instead, because users can use gen and offset to do some custom optimization.

Proposal

    pub fn query_position_to_coset(
        &self,
        query_pos: &[Boolean<F>],
        coset_dim: u64,
-    ) -> Result<Vec<FpVar<F>>, SynthesisError> {
+   ) -> Result<Self, SynthesisError> { 

---

For Admin Use

• [ ] Not duplicate issue
• [ ] Appropriate labels applied
• [ ] Appropriate contributors tagged
• [ ] Contributor assigned/self-assigned