AK: Make MappedFile heap-allocated and ref-counted

Let's adapt this class a bit better to how it's actually being used.

Instead of having valid/invalid states and storing an error in case
it's invalid, a MappedFile is now always valid, and the factory
function that creates it will return an OSError if mapping fails.

DevTools/UserspaceEmulator/Emulator.h

@@ -207,7 +207,7 @@ private:
     Optional<size_t> m_loader_text_size;
 
     struct CachedELF {
-        MappedFile mapped_file;
+        NonnullRefPtr<MappedFile> mapped_file;
         NonnullOwnPtr<Debug::DebugInfo> debug_info;
     };