core: Add new "mutable tree" class, use it for tar file import

The tar import code forced the resuscitation of a hackish "FileTree"
data type for representing an in-memory tree.  Split this out
into an OstreeMutableTree class for future use by any other in-memory
tree construction.

src/libostree/ostree.h

@@ -25,5 +25,6 @@
 #include <ostree-core.h>
 #include <ostree-repo.h>
 #include <ostree-checkout.h>
+#include <ostree-mutable-tree.h>
 
 #endif