Support serialization operations on big-endian platforms in little-endian format

[lemire]

Big-endian platforms are effectively gone. Nevertheless, as a matter of principle, we would like to support them for the IBM mainframe users out there.

Currently, if you are on a big-endian platform, all serialization (IO operations) are big endian, unlike what happens on virtually all other computers (where we serialize in little-endian format).

This means that you can use CRoaring on an IBM s390x mainframe, but the CRoaring file that you produce won't be compatible with the data files from common computers.

[lemire]

It is not a difficult problem, one just needs to flip the bytes around.