Add #pragma once or include guards in headers

[uselessgoddess]

sisharp-style 😀 https://github.com/linksplatform/Ranges/blob/f133625e55e874bdf433fb16cedb538cae43e564/cpp/Platform.Ranges/Platform.Ranges.h#L1

[Konard]

We should apply this decision in all files that were not generated by translator, as it does not support automatic generation of include statements yet.