This PR replaces UnitTest-cpp with Catch. Catch is downloaded
using cmake's ExternalProject_Add and installed into the
include directory as it's a header only library.
Signed-off-by: Rian Quinn <“rianquinn@gmail.com”>
* 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.
