* guix/store.scm (gc-action): New enumerate type.
(read-long-long, read-string-list, write-store-path,
write-store-path-list, read-store-path-list): New procedures.
(write-arg): Add support for `store-path' and `store-path-list'.
(read-arg): Add support for `store-path-list'.
(define-operation): Add support for multiple-value returns.
(run-gc, live-paths, dead-paths, collect-garbage, delete-paths): New
procedures.
(%long-long-max): New macro.
* tests/store.scm: New file.
* Makefile.am (TESTS): Add it.
