More APIs and documents for ECC operations in the circuit

[chancharles92]

• [ ] Add comments in the ECC circuits (e.g., MSM and other group operations) that the input points should be guaranteed to be well-formed (e.g. whether they're in the subgroup).
• [ ] Provide subgroup check circuit for points (useful for VRF application, etc..).

[chancharles92]

cc @alxiong @mrain