Use narrow_cast() in place of static_cast() in span.h and string_span.h

[neilmacintosh]

This will require a little rejiggering of headers and #include's, so I'm creating this issue to remind me to do it soon.

[TBBle]

Conversely, the existing narrow_cast in the as_span described as
// convert a span<const byte> to a span<const T>

seems to fail to be constexpr in Visual Studio 2015, while trying to compile the following (roughly)

using fixed_string_span = gsl::span<const gsl::byte, 0x20>;
fixed_string_span string0;
// Following line fails in VS2015, but passed in clang 3.7.0
auto charstring0 = as_span<const char>(string0);

2>gsl\span.h(1645): error C2131: expression did not evaluate to a constant
2> gsl\span.h(1645): note: failure was caused by call of undefined function or one not declared 'constexpr'
2> gsl\span.h(1645): note: see usage of 'gsl::narrow_cast'
2> src\AssetName.cpp(17): note: see reference to function template instantiation 'gsl::span<U,32> gsl::as_span<const char,32>(gsl::span<const gsl::byte,32>) noexcept' being compiled
2> with
2> [
2> U=const char
2> ]

That's complaining about the static_assert in that method,

    static_assert(
        std::is_trivial<std::decay_t<U>>::value &&
            (ConstByteSpan::bounds_type::static_size == dynamic_range ||
             ConstByteSpan::bounds_type::static_size % narrow_cast<std::ptrdiff_t>(sizeof(U)) == 0),
        "Target type must be a trivial type and its size must match the byte array size");

[neilmacintosh]

Hello @TBBle, the constexpr issue is most likely an outcome of the incomplete constexprsupport in Visual Studio 2015. While it is much better than it was, it still has a little way to go. For now, you sholud be able to leave constexpr #defined away for the GSL when using Visual Studio 2015. We all look forward to that changing soon.