[feat] Make selector compression optional

[jonathanpwang]

This PR does the following:

• Addresses Update serialization format according to upstream privacy-scaling-explorations/halo2#176 by adding a version byte to vkey serialization.
• Adds keygen_vk_custom function where user can choose whether to turn on selector compression(=combination) or not.
  • In halo2-axiom, keygen_vk will be set so the default is no selector compression.
• When selector compression is off, the VerifyingKey does not need to store anything about selectors. After keygen_vk_custom, the ConstraintSystem has entirely replaced selectors with fixed columns, and fixed commitments have been updated. The verify_proof function does not use selectors in any way. In keygen_pk you still need to know about selectors, but it reconstructs the ConstraintSystem from scratch anyways.
• I think I've modified the VerifyingKey::read and write functions accordingly, where there is now a new byte recording whether selector compression is on or off. When off, I skip serializing selectors since they are unnecessary for reasons mentioned above.