In key generation, figure out if we need l_0(X) computed more efficiently

[ed255]

In the key generation we find the following TODO: https://github.com/privacy-scaling-explorations/halo2/blob/73408a140737d8336490452193b21f5a7a94e7de/halo2_proofs/src/plonk/keygen.rs#L360-L365

We need to figure out:

1. Is it worth doing this part more efficiently? In my opinion, if this part represents a <1% of the key gen time, there's no need to improve it.
2. If we want to do it more efficiently, figure out how, and do it.