Specify BitOps_getBitString() bytes arg as const.

algorithms/encryption/bit_ops/src/bit_ops.(c, h)
	-Declare `BitOps_getBitString()`'s `bytes` argument as `const`,
	because it shouldn't be modified.
Add initial permutation of plaintext.

algorithms/encryption/des/src/des.(c, h)
	-Perform the initial permutation of the plaintext message.
	-Update `DES_encipher()` `key` argument with `const` keyword.