all: unify big.Int zero checks, use common/math in more places (#3716)

* common/math: optimize PaddedBigBytes, use it more

name              old time/op    new time/op    delta
PaddedBigBytes-8    71.1ns ± 5%    46.1ns ± 1%  -35.15%  (p=0.000 n=20+19)

name              old alloc/op   new alloc/op   delta
PaddedBigBytes-8     48.0B ± 0%     32.0B ± 0%  -33.33%  (p=0.000 n=20+20)

* all: unify big.Int zero checks

Various checks were in use. This commit replaces them all with Int.Sign,
which is cheaper and less code.

eg templates:

    func before(x *big.Int) bool { return x.BitLen() == 0 }
    func after(x *big.Int) bool  { return x.Sign() == 0 }

    func before(x *big.Int) bool { return x.BitLen() > 0 }
    func after(x *big.Int) bool  { return x.Sign() != 0 }

    func before(x *big.Int) int { return x.Cmp(common.Big0) }
    func after(x *big.Int) int  { return x.Sign() }

* common/math, crypto/secp256k1: make ReadBits public in package math

1 files changed, 2 insertions, 3 deletions

diff --git a/crypto/secp256k1/secp256_test.go b/crypto/secp256k1/secp256_test.go
index 287ab512e..f6582ecd5 100644
--- a/crypto/secp256k1/secp256_test.go
+++ b/crypto/secp256k1/secp256_test.go
@@ -24,6 +24,7 @@ import (
 	"encoding/hex"
 	"testing"
 
+	"github.com/ethereum/go-ethereum/common/math"
 	"github.com/ethereum/go-ethereum/crypto/randentropy"
 )
 
@@ -35,9 +36,7 @@ func generateKeyPair() (pubkey, privkey []byte) {
 		panic(err)
 	}
 	pubkey = elliptic.Marshal(S256(), key.X, key.Y)
-	privkey = make([]byte, 32)
-	readBits(privkey, key.D)
-	return pubkey, privkey
+	return pubkey, math.PaddedBigBytes(key.D, 32)
 }
 
 func randSig() []byte {