core: Add API to convert checksum -> csum in place

We already have the opposite, and this will be used in some
places to avoid a malloc.

https://bugzilla.gnome.org/show_bug.cgi?id=706031

src/libostree/ostree-core.h

@@ -132,6 +132,9 @@ char * ostree_checksum_from_bytes_v (GVariant *bytes);
 void ostree_checksum_inplace_from_bytes (const guchar *bytes,
                                          char         *buf);
 
+void ostree_checksum_inplace_to_bytes (const char *checksum,
+                                       guchar     *buf);
+
 const guchar *ostree_checksum_bytes_peek (GVariant *bytes);
 
 int ostree_cmp_checksum_bytes (const guchar *a, const guchar *b);