use binary format by default (except on macOS)

[tessus]

fixes #85

[Byron]

Thanks a lot! And I learned something as well, as I wasn't aware of std::env::consts (but used conditional compilation for these things as a binary is built per platform anyway).

[tessus]

You are welcome. Using conditional compilation is a mistake in most cases. e.g. for something like this it makes no sense.

Conditional compilation makes sense for libraries when the functions/methods are fundamentally different and are thousand of lines long.