store: 'references/substitutes' correctly handles the order of substitutes.

Before that, 'references/substitutes' would assume that
'substitutable-path-info' would return things in the same order as its
arguments, which is not the case.  Thus, it would sometimes provide
incorrect reference information, occasionally leading to infinite
loop (because dependency information would denote cycles.)

Fixes <http://bugs.gnu.org/22914>.
Reported by Eric Bavier <ericbavier@openmailbox.org>.

* guix/store.scm (references/substitutes): Make ITEMS the first argument
of the loop; match on it.  Use 'any' to find a matching substitute.
(substitutable-path-info): Clarify docstring about ordering.