Inconsistency with n4808

[Yaossg]

This library, as well as the implementation in gcc, is inconsistent with n4808, and surprisingly possesses the exact same mistakes as cppreference once did. I found some and fixed them in cppreference these days:

• missing noexcept for concat split simd_cast and static_simd_cast
• missing operator+ and operator~ for const_where_expression
• missing function split_by entirely