LibCore: Slightly rework the Core::Promise API

The previous iteration of this API was somewhat odd and rough in random
places, which degraded usability and made less than perfect sense.
This commit reworks the API to be a little closer to more
conventional promise APIs (a la javascript promises).

Also adds a test to ensure the class even works.

Tests/LibCore/CMakeLists.txt

@@ -1,10 +1,11 @@
 set(TEST_SOURCES
     TestLibCoreArgsParser.cpp
-    TestLibCoreFileWatcher.cpp
     TestLibCoreDeferredInvoke.cpp
-    TestLibCoreStream.cpp
     TestLibCoreFilePermissionsMask.cpp
+    TestLibCoreFileWatcher.cpp
+    TestLibCorePromise.cpp
     TestLibCoreSharedSingleProducerCircularQueue.cpp
+    TestLibCoreStream.cpp
 )
 
 foreach(source IN LISTS TEST_SOURCES)