Generator of Fr*

[ThomasPiellard]

Is your feature request related to a problem? Please describe.
In the fft package the method to set a generator of Fr* is not exposed and the comment is wrong e.g.

	// generator of the largest 2-adic subgroup

	domain.FrMultiplicativeGen.SetUint64(5)

for bn254 should be "generator of Fr* ". This generator might be needed e.g. for fflonk.

Describe the solution you'd like
Add a public method GenFrStar()