Add the ability to control which files in the input root are hardlinked

This change acts as a counter proposal to PR #216. Instead of globally
stating whether files should be hardlinked or not, it adds the ability
to let clients control whether files are aliased.

It achieves this by adding a salt field to FileNode. If two files have
the same salt and all other (non-name) properties match, they SHOULD get
constructed in the form of hardlinks.