formalize file/folder evaluations; use set-types for files/folders so that we can evaluate both file metadata & contents, rather than just assuming the contents; adjust slice view rule to just expand to the contents, rather than achieving the slice with a changed type

src/eval/eval_core.h

@@ -94,6 +94,7 @@ enum
 {
   E_SpaceKind_Null,
   E_SpaceKind_HashStoreKey,
+  E_SpaceKind_FileSystem,
   E_SpaceKind_FirstUserDefined,
 };