Add test module, func headers to des.h.

algorithms/encryption/des/src/(des.(c, h), test.c)
	-Add a `test` module, to contain the project's executable's main
	entry point so that the `des` module remains standalone code
	that can be easily compiled into another project.
	-Forward declare `DES_encipher()` and `DES_decipher()` in
	`des.h`.
	-Define dummy, placeholder `DES_encipher()`.