Change requirements from C++17 to C++11

[t-hishinuma]

C++17 is required by including header files. Change header files to work with C++11.

[t-hishinuma]

The same applies to nested namespaces. The example and test also need to be modified.

The monolish library is developed with C ++ 17, but only C ++ 11 is required for users installed with apt.

[t-hishinuma]

In the comment of the header, write "Header is written in C ++ 11"

[t-hishinuma]

done