core/vm, crypto/bn256: switch over to cloudflare library (#16203)

* core/vm, crypto/bn256: switch over to cloudflare library

* crypto/bn256: unmarshal constraint + start pure go impl

* crypto/bn256: combo cloudflare and google lib

* travis: drop 386 test job

crypto/bn256/cloudflare/gfp.go

@@ -0,0 +1,81 @@
+package bn256
+
+import (
+	"errors"
+	"fmt"
+)
+
+type gfP [4]uint64
+
+func newGFp(x int64) (out *gfP) {
+	if x >= 0 {
+		out = &gfP{uint64(x)}
+	} else {
+		out = &gfP{uint64(-x)}
+		gfpNeg(out, out)
+	}
+
+	montEncode(out, out)
+	return out
+}
+
+func (e *gfP) String() string {
+	return fmt.Sprintf("%16.16x%16.16x%16.16x%16.16x", e[3], e[2], e[1], e[0])
+}
+
+func (e *gfP) Set(f *gfP) {
+	e[0] = f[0]
+	e[1] = f[1]
+	e[2] = f[2]
+	e[3] = f[3]
+}
+
+func (e *gfP) Invert(f *gfP) {
+	bits := [4]uint64{0x3c208c16d87cfd45, 0x97816a916871ca8d, 0xb85045b68181585d, 0x30644e72e131a029}
+
+	sum, power := &gfP{}, &gfP{}
+	sum.Set(rN1)
+	power.Set(f)
+
+	for word := 0; word < 4; word++ {
+		for bit := uint(0); bit < 64; bit++ {
+			if (bits[word]>>bit)&1 == 1 {
+				gfpMul(sum, sum, power)
+			}
+			gfpMul(power, power, power)
+		}
+	}
+
+	gfpMul(sum, sum, r3)
+	e.Set(sum)
+}
+
+func (e *gfP) Marshal(out []byte) {
+	for w := uint(0); w < 4; w++ {
+		for b := uint(0); b < 8; b++ {
+			out[8*w+b] = byte(e[3-w] >> (56 - 8*b))
+		}
+	}
+}
+
+func (e *gfP) Unmarshal(in []byte) error {
+	// Unmarshal the bytes into little endian form
+	for w := uint(0); w < 4; w++ {
+		for b := uint(0); b < 8; b++ {
+			e[3-w] += uint64(in[8*w+b]) << (56 - 8*b)
+		}
+	}
+	// Ensure the point respects the curve modulus
+	for i := 3; i >= 0; i-- {
+		if e[i] < p2[i] {
+			return nil
+		}
+		if e[i] > p2[i] {
+			return errors.New("bn256: coordinate exceeds modulus")
+		}
+	}
+	return errors.New("bn256: coordinate equals modulus")
+}
+
+func montEncode(c, a *gfP) { gfpMul(c, a, r2) }
+func montDecode(c, a *gfP) { gfpMul(c, a, &gfP{1}) }