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 big endian issue in std.internal.math.biguintcore.d

The unit tests passes with this change 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.
Fix endian issue in std.bitmanip

BitArray uses an array of size_t elements but opEquals() and opCmp()
cast it to an array of ubyte elements. On a big endian architecture
the bytes are accessed in wrong order.
The solution is to always use size_t elements.
Merge pull request #1254 from redstar/endianbitmanip

Fix endian issue in std.bitmanip
Merge pull request #1249 from redstar/digestendian

Fix big endian version of MD5 and RipeMD160.
Merge pull request #1250 from redstar/bigintendian

Fix big endian issue in std.internal.math.biguintcore.d