Expose const and internal_repr getter fn.

[CPerezz]

• Rename S as TWO_ADACITY and expose it.
  Needed by Take fft_const from Bls lib plonk#152

• Implement internal_repr method which returns the internal representation of the Scalar reduced as [u64;4] so we can use any Scalar as an exponent in pow fn.
  This is needed by Test whether K1 and K2 form coset generators for BLS12-381 plonk#130 & Add a method to generate four or five coset generators, then hardcode them plonk#131