*: refactoring to support streams

when creating a manifest from, or validating, a stream like a tar
archive, it requires thinking about some of the functions differently
than walking a directory tree.

This is the beginning of allowing for such features.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

creator.go

@@ -0,0 +1,9 @@
+package mtree
+
+// dhCreator is used in when building a DirectoryHierarchy
+type dhCreator struct {
+	DH     *DirectoryHierarchy
+	curSet *Entry
+	curDir *Entry
+	curEnt *Entry
+}