LibCore: Make MappedFile a Stream

The internal reuse of FixedMemoryStream makes this straightforward.
There alread is one user of the new API, demonstrating the need for this
change beyond what I said out to use it for :^)

Tests/LibCore/CMakeLists.txt

@@ -3,6 +3,7 @@ set(TEST_SOURCES
     TestLibCoreDeferredInvoke.cpp
     TestLibCoreFilePermissionsMask.cpp
     TestLibCoreFileWatcher.cpp
+    TestLibCoreMappedFile.cpp
     TestLibCorePromise.cpp
     TestLibCoreSharedSingleProducerCircularQueue.cpp
     TestLibCoreStream.cpp