core: Clean up checksumming

Don't expose GChecksum in APIs.  Add a new stream class which allows
us to pass an input stream somewhere, but gather a checksum as it's
read.

Move some bits of the internals towards binary csums.

src/libostree/ostree.h

@@ -22,6 +22,7 @@
 
 #ifndef __OSTREE_H__
 
+#include <ostree-checksum-input-stream.h>
 #include <ostree-core.h>
 #include <ostree-repo.h>
 #include <ostree-mutable-tree.h>