strict_not_null for unique_ptr (#1179)

- `strict_not_null<std::unique_ptr<int>>{ std::make_unique<int>()}` failed to compile
  - `strict_not_null` ctor needs to move the passed `unique_ptr`, not copy
  - Copied `not_null` `TestNotNullConstructors` for `strict_not_null`
- The `noexcept` specifiers on the `strict_not_null` and `not_null` constructors were out of sync.
- Added unit test for `not_null<unique_ptr<T>>` and for `strict_not_null<unique_ptr<T>>`
- Added unit test for `gsl::swap` for two `strict_not_null`
- Added unit test for `gsl::swap` for `not_null` and `strict_not_null`

Co-authored-by: Werner Henze <w.henze@avm.de>

include/gsl/pointers

@@ -275,19 +275,19 @@ class strict_not_null : public not_null<T>
 {
 public:
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr explicit strict_not_null(U&& u) : not_null<T>(std::forward<U>(u))
+    constexpr explicit strict_not_null(U&& u) noexcept(std::is_nothrow_move_constructible<T>::value) : not_null<T>(std::forward<U>(u))
     {}
 
     template <typename = std::enable_if_t<!std::is_same<std::nullptr_t, T>::value>>
-    constexpr explicit strict_not_null(T u) : not_null<T>(u)
+    constexpr explicit strict_not_null(T u) noexcept(std::is_nothrow_move_constructible<T>::value) : not_null<T>(std::move(u))
     {}
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr strict_not_null(const not_null<U>& other) : not_null<T>(other)
+    constexpr strict_not_null(const not_null<U>& other) noexcept(std::is_nothrow_move_constructible<T>::value) : not_null<T>(other)
     {}
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
-    constexpr strict_not_null(const strict_not_null<U>& other) : not_null<T>(other)
+    constexpr strict_not_null(const strict_not_null<U>& other) noexcept(std::is_nothrow_move_constructible<T>::value) : not_null<T>(other)
     {}
 
     // To avoid invalidating the "not null" invariant, the contained pointer is actually copied