pgsql: Refactor int128.h, bringing the native and non-native code toget

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(dot)a(dot)rasheed(at)gmail(dot)com>
Reviewed-by: John Naylor <johncnaylorls(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAEZATCWgBMc9ZwKMYqQpaQz2X6gaamYRB+RnMsUNcdMcL2Mj_w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5761d991c98471b7dab3147b71f9caf00a6f8ce4

Modified Files
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src/include/common/int128.h | 112 +++++++++++++++++---------------------------
1 file changed, 42 insertions(+), 70 deletions(-)