write_xaiger: Get by without endianness helpers

[povik]

What are the reasons/motivation for this change?

A clean-up

Explain how this is achieved.

Avoid any bswap helpers or the detection of host endianness when there's a portable way to write out a number in big endian: e.g. for 32-bit x you can do an equivalent of put(x >> 24); put(x >> 16); put(x >> 8); put(x);

[widlarizer]

Looks good