Correct packing/ unpacking Matrices modulo Q ( = 2^D )

[itzmeanjan]

Correct logic of packing/ unpacking matrices to/ from byte arrays, following algorithm 3/ 4.

Note Previously I was interpreting elements of matrices modulo Q in little-endian byte order, though if we carefully go through algorithm 3/ 4, we'll see we should actually interpret bytes in big-endian order.

Run tests, by issuing

make