Add support for using C++17 string_view or a fallback

Define a `cm::string_view` type implemented via C++17 `std::string_view`
when available.  Provide a fallback implementation for C++11 and C++14
compilers.

The fallback implementation was written by reading documentation of the
standard spec.  We have no dedicated tests for it, but it will be
covered by tests of its clients later.