Use a `Polynomial` type instead of `Vec<E::Fr>`

[osuketh]

Operations over polynomial and commitment would be changed because of the new type definition. https://github.com/LayerXcom/zero-chain/blob/d866d2e469ff4c9ee3b58db061fe25e71fbd765f/core/sonic/src/polynomials/mod.rs#L6

[osuketh]

It could be type alias.

[burdges]

You might want +, *, etc. defined, which prevents using a type alias. You might check into using both Vec and SmallVec with roughly:

pub struct Polynomial<E,V>(V)
where E: Engine: V: Borrow<[E::Fr]>+BorrowMut<[E::Fr]>+Default; 

Appears VecLike was deprecated in favor of just Extend though, so maybe mutation does not work so well.

[osuketh]

LGTM! It should be implemented as operator overloadings. I've never used SmallVec. How much would it be optimized actually?