memoization: Profiling support keeps track of lookups and hits.

* guix/memoization.scm (<cache>): New record type.
(define-lookup-procedure, define-update-procedure): New macros.
(cache-ref, cacheq-ref, cache-set!, cacheq-set!): New procedures.
(cached/mv, cachedq/mv, cached, cachedq): Use them instead of 'hash-ref'
and 'hash-set!'.
(%make-hash-table*): When 'profiled?' returns true, return a <cache>
object.
(define-cache-procedure): Adjust to show cache lookups and hits.