* Improve const correctness in string_span
* Improve const correctness in bounds_tests.cpp and byte_tests.cpp
* Improve const correctness in span_tests.cpp
* Improve const correctness in utils_tests.cpp
* Use gsl::owner for dynamically allocated memory in string_span_tests.cpp
* Improve const correctness in string_span_tests.cpp
* Improve const correctness for strided_span_tests.cpp
* Nest "gsl" directory inside a new "include" directory.
* Cleanup the _MSC_VER conditionals a bit; use constexpr on VS2017+.
* Don't #define noexcept on non-Microsoft implementations.
* Workaround VS2017 bug in multi_span. (Also implement == and != for static_bounds_dynamic_range_t because I'm an EoP semantic soundness snob.)
Fixes#441.
