Merge topic 'custom-string'
2d68b2c593 String: Add str_if_stable() as a const alternative to str()
a0841b59bd String: Add support for a ""_s string literal syntax
9d5fe8e96a String: Add 'borrow' member to construct borrowing instances
80802a002c String: Add support for concatenation by operator+
ff69763ca0 String: Add a custom string type
410a3e4b22 Add support for using C++17 string_view or a fallback
Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Daniel Pfeifer <daniel@pfeifer-mail.de>
Acked-by: Pavel Solodovnikov <hellyeahdominate@gmail.com>
Merge-request: !2578
