introduce gsl::swap for swapping gsl::not_null (#1160)

fixes: https://github.com/microsoft/GSL/issues/1129

* create gsl::swap<T>(T&, T&) which wraps std::swap
* specialize gsl::swap<T>(gsl::not_null<T>&, gsl::not_null<T>&)
* add tests

tests/pointers_tests.cpp

@@ -0,0 +1,21 @@
+#include <gtest/gtest.h>
+
+#include <gsl/pointers>
+
+#include <memory>
+
+TEST(pointers_test, swap)
+{
+    // taken from gh-1129:
+    gsl::not_null<std::unique_ptr<int>> a(std::make_unique<int>(0));
+    gsl::not_null<std::unique_ptr<int>> b(std::make_unique<int>(1));
+
+    EXPECT_TRUE(*a == 0);
+    EXPECT_TRUE(*b == 1);
+
+    gsl::swap(a, b);
+
+    EXPECT_TRUE(*a == 1);
+    EXPECT_TRUE(*b == 0);
+}
+