refactor polynomial operations

[ashWhiteHat]

I refactored polynomial operations because it was a little bit of complicated.
The reason why I removed interpolate was that I would like to work on removal of Evaluations::domain as follows and interpolate would be the bottleneck of it.
https://github.com/dusk-network/plonk/blob/master/src/fft/evaluations.rs#L39

I would appreciate it if you could confirm.
Thank you.

[ashWhiteHat]

I also think that we should integrate fft::evaluations.rs and fft::polynomial.rs.
Polynomial has two representation Coefficient and Point-Value.
fft::evaluations.rs is Point-Value form of Polynomial thus it should be in fft::polynomial.rs.
Evaluations::domain is only used for domain equation so we can remove it.
Then, fft::evaluations.rs has a few code so we can move it to fft::polynomial.rs.

Polynomial Representation
https://web.cecs.pdx.edu/~maier/cs584/Lectures/lect07b-11-MG.pdf