LibCore+AK: Move MappedFile from AK to LibCore

MappedFile is strictly a userspace thing, so it doesn't belong in AK
(which is supposed to be user/kernel agnostic.)

Userland/Applications/Help/ManualModel.cpp

@@ -72,7 +72,7 @@ ErrorOr<StringView> ManualModel::page_view(String const& path) const
             return StringView { mapped_file.value()->bytes() };
     }
 
-    auto file = TRY(MappedFile::map(path));
+    auto file = TRY(Core::MappedFile::map(path));
 
     StringView view { file->bytes() };
     m_mapped_files.set(path, move(file));