store: Allow objects in the cache to be inserted and search for with 'equal?'.

* guix/store.scm (cache-object-mapping): Add #:vhash-cons parameter and
honor it.
(lookup-cached-object): Add #:vhash-fold* parameter and honor it.
(%mcached): Add #:vhash-fold* and #:vhash-cons and honor them.
(mcached): Add clauses with 'eq?' and 'equal?' as the first argument.