Fix big endian version of MD5 and RipeMD160.

Obviously the code has never been run on a big endian architecuture.
With these changes the unit tests passes on Linux/PPC64.
Fix endian issues in std.md5

I replaced the methods Encode and Decode with calls to methods
from std.bitmanip. This solves the endian issues.
Merge pull request #1249 from redstar/digestendian

Fix big endian version of MD5 and RipeMD160.