fix "type ‘struct uint128’ violates the C++ One Definition Rule" warning

should also fix issues #32 and #33

The class uint128 has two possible implementations, depending on the macro __SIZEOF_INT128__. Though, unit tests where designed to test always the implementation that is picked when __SIZEOF_INT128__ is undefined. Undefining the macro __SIZEOF_INT128__ can lead to violation of the C++ One Definition Rule in unit tests. The unit tests and the implementation of the class uint128 were revised to test always both possible implementations without violating the One Definition Rule.