Update bits.go (#4055)

Author: soniseth0

util/arbmath/bits.go

@@ -21,7 +21,11 @@ func FlipBit(data bytes32, bit byte) bytes32 {
 
 // ConcatByteSlices unrolls a series of slices into a singular, concatenated slice
 func ConcatByteSlices(slices ...[]byte) []byte {
-	unrolled := []byte{}
+	totalLen := 0
+	for _, slice := range slices {
+		totalLen += len(slice)
+	}
+	unrolled := make([]byte, 0, totalLen)
 	for _, slice := range slices {
 		unrolled = append(unrolled, slice...)
 	}