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-core.h

@@ -136,6 +136,8 @@ void ostree_checksum_inplace_to_bytes (const char *checksum,
 
 const guchar *ostree_checksum_bytes_peek (GVariant *bytes);
 
+const guchar *ostree_checksum_bytes_peek_validate (GVariant *bytes, GError **error);
+
 int ostree_cmp_checksum_bytes (const guchar *a, const guchar *b);
 
 gboolean ostree_validate_rev (const char *rev, GError **error);