adds C++20 module interface sus

`sus` is an experimental module interface unit. `import sus;` replaces
including most conventional Subspace headers. Users will need to include
headers suffixed with `_macros.h` in order to use Subspace modules, but
should not include any headers otherwise.

Author: cjdb

CMakeLists.txt

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-cmake_minimum_required(VERSION 3.16)
+cmake_minimum_required(VERSION 3.30)
 project(subspace)
 
 # compile_commands.json is needed for Subdoc's documentation generation.