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, 1 insertions, 1 deletions

diff --git a/tests/util.go b/tests/util.go
index 78bb06d06..7a08e5ed8 100644
--- a/tests/util.go
+++ b/tests/util.go
@@ -71,7 +71,7 @@ func checkLogs(tlog []Log, logs []*types.Log) error {
 					}
 				}
 			}
-			genBloom := common.LeftPadBytes(types.LogsBloom([]*types.Log{logs[i]}).Bytes(), 256)
+			genBloom := math.PaddedBigBytes(types.LogsBloom([]*types.Log{logs[i]}), 256)
 
 			if !bytes.Equal(genBloom, common.Hex2Bytes(log.BloomF)) {
 				return fmt.Errorf("bloom mismatch")