Add LagrangeBasis trait

[adria0]

Currently Basis can refer to coefficients, lagrange basis or extended lagrange basis.
Some functions like

halo2/halo2_backend/src/plonk/evaluation.rs

Line 836 in 0b75a92

pub fn evaluate<F: Field, B: Basis>(

does not make sense for coeffients. Needs to work only with lagrange.

the proposal is to add a new trait LagrangeBasis

pub trait LagrangeBasis: Copy + Debug + Send + Sync {}
impl LagrangeBasis for LagrangeCoeff {}
impl LagrangeBasis for ExtendedLagrangeCoeff {}

to use them for Lagrange basis polynomials.

[CPerezz]

I wouldn't dive too much on re-structuring the polynomial-related things here as we want to eventually migrate all the polynomial functionallity into a different repo/crate.

Just for info.

[adria0]

Just a minor adjustment to improve correctness, that's all.

[ed255]

This is addressed via #289 right?

[adria0]

Done in #289