refactor: clean iop APIs + add Marshal methods

[gbotrel]

iop package methods / functions are more consistent, and takes as argument / return the same type *iop.Polynomial .

There is now only Polynomial (previously WrappedPolynomial); the contract of this struct is consistent;

it is initialized with a pointer to a slice of field elements (the coefficients: *[]fr.Element) and any calls on Polynomial methods will mutate the original slice (even if it needs to grow for a FFT).

The method Clone() *Polynomial performs a deep-copy and clones the underlying vector of coefficients.

The method ShallowClone() *Polynomial returns a new object with the same underlying coefficients; this allows modification of the shift size and blindedSize attributres (Blind method for example) without re-allocating.

Additionally, Polynomial now implements io.ReaderFrom and io.WriterTo.

• fix: vector marshal methods on pointer type
• feat: support for Vector encode / decode in Marshal
• refactor: merge Polynomial and WrappedPolynomial
• refactor: make Coefficients private and expose Slice access
• recfactor: fix constructor with []fr.Element input
• remove more apis with unexposed struct
• remove more apis with unexposed struct
• remove more apis with unexposed struct
• refactor: most consistent APIs
• style :code cleaning
• refactor: resize -> grow
• feat: added WriterTo and ReaderFrom to Polynomial