Cleaning finite field generators

[ThomasPiellard]

when using a finite field (e.g. "../bn254/fr"):

• fr.Generator should be renamed fr.GenBiggestTwoAdicSubgroup or something explaining exactly what the generator in question is
• GeneratorFullMultiplicativeGroup, which is in the FFT package for now, should be in the finite fields package, it should be named fr.GeneratorMultiplicativeGroup() or just fr.Generator (this time there is no ambiguity about what the generator does)

Those little modifications would help cleaning the templates for the fft code generation (which should depend only on the FieldConfig) and would clarify the api