Refactor int128.h, bringing the native and non-native code together.

Refactor int128.h, bringing the native and non-native code together.

This rearranges the code in src/include/common/int128.h, so that the
native and non-native implementations of each function are together
inside the function body (as they are in src/include/common/int.h),
rather than being in separate parts of the file.

This improves readability and maintainability, making it easier to
compare the native and non-native implementations, and avoiding the
need to duplicate every function comment and declaration.

Author: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Discussion: https://postgr.es/m/CAEZATCWgBMc9ZwKMYqQpaQz2X6gaamYRB+RnMsUNcdMcL2Mj_w@mail.gmail.com