Add support for multiple inputs.

* Makefile.am (dist_sql_DATA): Add src/sql/upgrade-1.sql.
* bin/cuirass.in (show-help, %options, main): Remove the LOAD-PATH option that
was used afterwards as %GUIX-PACKAGE-PATH.
* bin/evaluate.in (absolutize, input-checkout, spec-source, spec-load-path,
spec-package-path, format-checkouts): New procedures.
(%not-colon): Remove variable.
(main): Take the load path, package path and PROC from the checkouts that
result from the inputs.  Format the checkouts before sending them to the
procedure.  Remove the LOAD-PATH argument.
* doc/cuirass.texi (Overview, Database schema): Document the changes.
* examples/{guix-jobs.scm, hello-git.scm, hello-singleton.scm,
hello-subset.scm, random.scm}: Adapt to the new specification format.
* examples/guix-track-git.scm (package->spec): Rename to PACKAGE->INPUT.
(package->git-tracked): Replace FETCH-REPOSITORY with FETCH-INPUT and handle
the new format of its return value.
* examples/random-jobs.scm (make-random-jobs): Rename RANDOM to CHECKOUT.
Rename the checkout from 'random (which is a specification) to 'cuirass (which
is a checkout resulting from an input).
* src/cuirass/base.scm (fetch-repository): Rename to fetch-input.  Rename SPEC
to INPUT.  Return a checkout object instead of returning two values.
(evaluate): Take a list of CHECKOUTS and COMMITS as arguments, instead of
SOURCE.  Remove TOKENIZE and LOAD-PATH.  Pass the CHECKOUTS instead of the
SOURCE to "evaluate".  Remove %GUIX-PACKAGE-PATH.  Build the EVAL object
instead of getting it from "evaluate".
(compile?, fetch-inputs, compile-checkouts): New procedures.
(process-specs): Fetch all inputs instead of only fetching one repository.
The result of that fetching operation is a list of CHECKOUTS whose COMMITS are
used as a STAMP.
(%guix-package-path, set-guix-package-path): Remove them.
* src/cuirass/database.scm (db-add-input, db-get-inputs): New procedures.
(db-add-specification, db-get-specifications): Adapt to the new specification
format.  Add/get all inputs as well.
(db-add-evaluation): Rename REVISION to COMMITS.  Store COMMITS as space
separated commit hashes.
(db-get-builds): Rename REPO_NAME to NAME.
(db-get-stamp): Rename COMMIT to STAMP.  Return #f when there is no STAMP.
(db-add-stamp): Rename COMMIT to STAMP.  Deal with DB-GET-STAMP's new return
value.
(db-get-evaluations): Rename REVISION to COMMITS.  Tokenize COMMITS.
* src/cuirass/utils.scm (%non-blocking): Export it.
* src/schema.sql (Inputs): New table that refers to the Specifications table.
(Specifications): Move input related fields to the Inputs table.  Rename
REPO_NAME to NAME.  Rename ARGUMENTS to PROC_ARGS.  Rename FILE to PROC_FILE.
Add LOAD_PATH_INPUTS, PACKAGE_PATH_INPUTS and PROC_INPUT fields that refer to
the Inputs table.
(Stamps): Rename REPO_NAME to NAME.
(Evaluations): Rename REPO_NAME to NAME.  Rename REVISION to COMMITS.
(Specifications_index): Replace with Inputs_index.
* src/sql/upgrade-1.sql: New file.
* tests/database.scm (example-spec, make-dummy-eval, sqlite-exec): Adapt to
the new specifications format.  Rename REVISION to COMMITS.
* tests/http.scm (evaluations-query-result, fill-db): Idem.