* guix/scripts/substitute-binary.scm: New file.
* Makefile.am (MODULES): Add it.
* nix/scripts/substitute-binary.in: New file.
* config-daemon.ac: Produce nix/scripts/substitute-binary.
* daemon.am (nodist_pkglibexec_SCRIPTS): Add
nix/scripts/substitute-binary.
* guix/store.scm (substitutable-path-info): Use the
`query-substitutable-path-infos' RPC.
* nix/nix-daemon/guix-daemon.cc (main): Honor `NIX_SUBSTITUTERS'.
* pre-inst-env.in: Set `NIX_SUBSTITUTERS'.
* test-env.in: Leave `NIX_SUBSTITUTERS' unchanged. Set
`GUIX_BINARY_SUBSTITUTE_URL, and create
$NIX_STATE_DIR/substituter-data.
Run `guix-daemon' within `./pre-inst-env'.
* tests/store.scm ("substitute query"): New test.
* 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.