Start handling ids as numbers, rather than strings

squee, returns all data as strings, and expects strings as inputs to
queries. So, keeping the ids as strings was easy initially, but it means that
you can't tell from the type whether it should be quoted, or not...

Therefore, handle ids as strings, converting them to numbers when they're
fetched from the database, and back to strings as part of the queries.

tests/model-package-metadata.scm

@@ -58,7 +58,7 @@
                 (list mock-inferior-package-foo
                       mock-inferior-package-foo-2)
                 (test-license-set-ids conn))
-             ((x) (string? x))))
+             ((x) (number? x))))
          #:always-rollback? #t))
 
       (with-postgresql-transaction