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

crypto/secp256k1/secp256.go

@@ -38,7 +38,6 @@ import "C"
 
 import (
 	"errors"
-	"math/big"
 	"unsafe"
 )
 
@@ -129,16 +128,3 @@ func checkSignature(sig []byte) error {
 	}
 	return nil
 }
-
-// reads num into buf as big-endian bytes.
-func readBits(buf []byte, num *big.Int) {
-	const wordLen = int(unsafe.Sizeof(big.Word(0)))
-	i := len(buf)
-	for _, d := range num.Bits() {
-		for j := 0; j < wordLen && i > 0; j++ {
-			i--
-			buf[i] = byte(d)
-			d >>= 8
-		}
-	}
-}