Catch was being installed to the root of the cmake project. This
violated the source code tree itself and would not work at all if GSL
was being consumed with add_subdirectory.
CMake will now install catch to the build tree under /dependencies.
* 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.
