common/hexutil: improve performance of EncodeBig (#23780)

- use Text instead of fmt.Sprintf
- reduced allocs from 6 to 2
- improved speed

common/hexutil/hexutil.go

@@ -176,13 +176,14 @@ func MustDecodeBig(input string) *big.Int {
 }
 
 // EncodeBig encodes bigint as a hex string with 0x prefix.
-// The sign of the integer is ignored.
 func EncodeBig(bigint *big.Int) string {
-	nbits := bigint.BitLen()
-	if nbits == 0 {
+	if sign := bigint.Sign(); sign == 0 {
 		return "0x0"
+	} else if sign > 0 {
+		return "0x" + bigint.Text(16)
+	} else {
+		return "-0x" + bigint.Text(16)[1:]
 	}
-	return fmt.Sprintf("%#x", bigint)
 }
 
 func has0xPrefix(input string) bool {