* Fix endianness bug in write_digit2_separated

* Move endianness check to compile time if it possible

* Turn 8 into a constant

this fixes a breaking ABI change that was introduce in the upgrade from
8.0.1 to 8.1.0.

Fixes #2695.

Fixes issue #2691

I got warnings from -Wnoexcept, fixed them.

to make sure that error tests do not fail because they become outdated

since it takes too much time to complete, similar to other tests with additional
CMake invocation