Move commitment serialization to helper function

[anupsv]

Move the commitment serialization to a helper function so it's cleaner.

[bxue-l2]

where does it show up? if it is pointed out, then someone else could take the work token

[anupsv]

3 places.

• https://github.com/Layr-Labs/rust-kzg-bn254/blob/master/src/kzg.rs#L988
• https://github.com/Layr-Labs/rust-kzg-bn254/blob/master/src/kzg.rs#L1246
• https://github.com/Layr-Labs/rust-kzg-bn254/blob/master/src/kzg.rs#L1267