Initial basic static delta code drop

This has a very basic level of functionality (deltas can be generated,
and applied offline).  There is only some stubbed out pull code to
fetch them via HTTP.

But, better to commit this now and improve it from a known starting
point, rather than have it languish in a branch.

src/libostree/ostree-varint.h

@@ -24,9 +24,10 @@
 
 G_BEGIN_DECLS
 
-guint64 _ostree_read_varuint64 (const guint8   *buf,
-                                gsize           buflen,
-                                gsize          *bytes_read);
+gboolean _ostree_read_varuint64 (const guint8   *buf,
+                                 gsize           buflen,
+                                 guint64        *out_value,
+                                 gsize          *bytes_read);
 
 void _ostree_write_varuint64 (GString *buf, guint64 n);