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Syntax sugar: user-defined literals for arg and str

A few smaller fixes and a test output improvement

It used to pass anyway because the expected output was identical
to the Python set.

- Conversion warning on clang: 'long' to 'size_t'
- Unused variable warning on MSVC

Fix Cygwin support

Simplify CMake build using add_subdirectory

Update document version to the next release 1.8