use Vital.Bitwise.xor instead of built-in xor() function

Because built-in bitwise operation functions are introduced at 7.3.377
and they are not available before 7.3.377. So I use Vital.Bitwise.xor
instead of built-in one, which switches built-in function and an
original implementation.
Merge pull request #110 from rhysd/improve/xor128_compatibility

use Vital.Bitwise.xor instead of built-in xor() function