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

2 files changed, 2 insertions, 2 deletions

diff --git a/light/state_object.go b/light/state_object.go
index 03d4868cd..d023270d5 100644
--- a/light/state_object.go
+++ b/light/state_object.go
@@ -202,7 +202,7 @@ func (self *StateObject) Copy() *StateObject {
 
 // empty returns whether the account is considered empty.
 func (self *StateObject) empty() bool {
-	return self.nonce == 0 && self.balance.BitLen() == 0 && bytes.Equal(self.codeHash, emptyCodeHash)
+	return self.nonce == 0 && self.balance.Sign() == 0 && bytes.Equal(self.codeHash, emptyCodeHash)
 }
 
 // Balance returns the account balance
diff --git a/light/txpool.go b/light/txpool.go
index 365f02d25..ecd6cfa26 100644
--- a/light/txpool.go
+++ b/light/txpool.go
@@ -365,7 +365,7 @@ func (pool *TxPool) validateTx(ctx context.Context, tx *types.Transaction) error
 	// Transactions can't be negative. This may never happen
 	// using RLP decoded transactions but may occur if you create
 	// a transaction using the RPC for example.
-	if tx.Value().Cmp(common.Big0) < 0 {
+	if tx.Value().Sign() < 0 {
 		return core.ErrNegativeValue
 	}