Fixes #660: unique_span::operator= correction

* The operator now properly returns `*this`.
* No longer assigning to `size()` and `data()`, but rather to `*this`-as-a-span.
* Added an explicit instantiation of a unique_span to the example program we compile with different versions of the C++ standard.

examples/other/new_cpp_standard/main.cpp

@@ -23,6 +23,14 @@ cuda::device::id_t get_current_device_id()
     return device.id();
 }
 
+void unique_spans()
+{
+	cuda::memory::host::unique_span<float> data1(nullptr, 0);
+	cuda::memory::host::unique_span<float> data2(nullptr, 0);
+
+	data1 = std::move(data2);
+}
+
 int main() 
 {
 	auto count = cuda::device::count();
@@ -38,5 +46,6 @@ int main()
 	auto nvtx_color_yellow = cuda::profiling::color_t::from_hex(0x0FFFF00);
 	(void) nvtx_color_yellow;
 #endif
+
 	std::cout << "SUCCESS\n";
 }

src/cuda/api/detail/unique_span.hpp

@@ -106,8 +106,8 @@ class unique_span : public ::cuda::span<T> {
 		if (data() != nullptr) {
 			deleter_type{}(data());
 		}
-		data() = released.data();
-		size() = released.size();
+		static_cast<span_type&>(*this) = released;
+		return *this;
 	}
 
 	/// No plain dereferencing - as there is no guarantee that any object has been