*: loads of walking features

For the most part, all the keywords for a standard mtree spec now have a
function to produce the contents for a creator.

These are used in the "walk" function, and will be used next in the
"check" logic.

This is still a WIP, as the DirectoryHierarchy produced from the current
Walk() is not all-together a valid document.

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

check.go

@@ -0,0 +1,8 @@
+package mtree
+
+type Result struct {
+}
+
+func Check(root string, dh *DirectoryHierarchy) (*Result, error) {
+	return nil, nil
+}