Cleanup include structure, constexpr and noexcept compiler workarounds.

* 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.

include/gsl/gsl_algorithm

@@ -0,0 +1,59 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
+//
+// This code is licensed under the MIT License (MIT).
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#pragma once
+
+#ifndef GSL_ALGORITHM_H
+#define GSL_ALGORITHM_H
+
+#include <gsl/span>
+#include <algorithm>
+
+#ifdef _MSC_VER
+ #pragma warning(push)
+
+ // turn off some warnings that are noisy about our Expects statements
+ #pragma warning(disable : 4127) // conditional expression is constant
+
+ // blanket turn off warnings from CppCoreCheck for now
+ // so people aren't annoyed by them when running the tool.
+ // more targeted suppressions will be added in a future update to the GSL
+ #pragma warning(disable : 26481 26482 26483 26485 26490 26491 26492 26493 26495)
+#endif // _MSC_VER
+
+namespace gsl
+{
+
+template <class SrcElementType, std::ptrdiff_t SrcExtent,
+          class DestElementType, std::ptrdiff_t DestExtent>
+void copy(span<SrcElementType, SrcExtent> src, span<DestElementType, DestExtent> dest)
+{
+    static_assert(std::is_assignable<decltype(*dest.data()), decltype(*src.data())>::value,
+                  "Elements of source span can not be assigned to elements of destination span");
+    static_assert(SrcExtent == dynamic_extent || DestExtent == dynamic_extent || (SrcExtent <= DestExtent),
+                  "Source range is longer than target range");
+
+    Expects(dest.size() >= src.size());
+    std::copy_n(src.data(), src.size(), dest.data());
+}
+
+} // namespace gsl
+
+#ifdef _MSC_VER
+ #pragma warning(pop)
+#endif // _MSC_VER
+
+#endif // GSL_ALGORITHM_H