Extend VirtualFilesystem's file mapping methods

[DissectBot]

Currently the .map_file() and .map_file_fh() methods only support mapping file entries. This can be extended to also mapping entries representing directories. This would require the creation of a MappedDirectory class.

On top of that, it might be useful to have a similar way to map symlinks. This is a bit trickier as currently the difference between a file and directory is indicated by a final / in the vfspath parameter.