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, 2 deletions

diff --git a/crypto/signature_cgo.go b/crypto/signature_cgo.go
index 5faa6061f..d1485de08 100644
--- a/crypto/signature_cgo.go
+++ b/crypto/signature_cgo.go
@@ -23,7 +23,7 @@ import (
 	"crypto/elliptic"
 	"fmt"
 
-	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/math"
 	"github.com/ethereum/go-ethereum/crypto/secp256k1"
 )
 
@@ -53,7 +53,7 @@ func Sign(hash []byte, prv *ecdsa.PrivateKey) (sig []byte, err error) {
 	if len(hash) != 32 {
 		return nil, fmt.Errorf("hash is required to be exactly 32 bytes (%d)", len(hash))
 	}
-	seckey := common.LeftPadBytes(prv.D.Bytes(), prv.Params().BitSize/8)
+	seckey := math.PaddedBigBytes(prv.D, prv.Params().BitSize/8)
 	defer zeroBytes(seckey)
 	return secp256k1.Sign(hash, seckey)
 }