Add polynomials arithmetic in goff

Polynomials and ifft moved to goff (iden3/go-iden3-crypto/ff) instead of *big.Int.

Benchmarks:

- Before:
BenchmarkArithmetic/polynomialSub-4         	    2774	    441063 ns/op
BenchmarkArithmetic/polynomialMul-4         	       1	1135732757 ns/op
BenchmarkArithmetic/polynomialDiv-4         	     768	   1425192 ns/op
BenchmarkGenerateProof-4                    	       1	2844488975 ns/op

- With this commit:
BenchmarkArithmetic/polynomialSubE-4        	   23097	     54152 ns/op
BenchmarkArithmetic/polynomialMulE-4        	      25	  44914327 ns/op
BenchmarkArithmetic/polynomialDivE-4        	    8703	    132573 ns/op
BenchmarkGenerateProof-4                    	       1	1530398526 ns/op

prover/arithmetic_test.go

@@ -0,0 +1,66 @@
+package prover
+
+import (
+	"crypto/rand"
+	"math/big"
+	"testing"
+
+	cryptoConstants "github.com/iden3/go-iden3-crypto/constants"
+	"github.com/iden3/go-iden3-crypto/utils"
+)
+
+func randBI() *big.Int {
+	maxbits := 253
+	b := make([]byte, (maxbits/8)-1)
+	_, err := rand.Read(b)
+	if err != nil {
+		panic(err)
+	}
+	r := new(big.Int).SetBytes(b)
+	return new(big.Int).Mod(r, cryptoConstants.Q)
+}
+
+func BenchmarkArithmetic(b *testing.B) {
+	// generate arrays with bigint
+	var p, q []*big.Int
+	for i := 0; i < 1000; i++ {
+		pi := randBI()
+		p = append(p, pi)
+	}
+	for i := 1000 - 1; i >= 0; i-- {
+		q = append(q, p[i])
+	}
+	pe := utils.BigIntArrayToElementArray(p)
+	qe := utils.BigIntArrayToElementArray(q)
+
+	b.Run("polynomialSub", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			polynomialSub(p, q)
+		}
+	})
+	b.Run("polynomialSubE", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			polynomialSubE(pe, qe)
+		}
+	})
+	b.Run("polynomialMul", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			polynomialMul(p, q)
+		}
+	})
+	b.Run("polynomialMulE", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			polynomialMulE(pe, qe)
+		}
+	})
+	b.Run("polynomialDiv", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			polynomialDiv(p, q)
+		}
+	})
+	b.Run("polynomialDivE", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			polynomialDivE(pe, qe)
+		}
+	})
+}