Impl. final permutation in _expansionPermutation().

algorithms/encryption/des/src/des.(c, h)
	-Redefine `_expansionPermutation()` to accept another argument,
	the target block into which its expanded block will be XOR'ed.
	This allows us to perform the XOR from inside
	`_expansionPermutation()` with a local operand, instead of
	returning a pointer to it (and thus having to allocate/free
	memory, etc).
	-Implement final permutation in `_expansionPermutation()`.