Add supporting tools for the GNU Build System.

* guix/derivations.scm (build-expression->derivation): Add all of INPUTS
  as inputs to the final derivation.

* guix/build/gnu-build-system.scm, guix/build/utils.scm,
  guix/gnu-build-system.scm: New files.

* tests/builders.scm ("gnu-build"): New test.

Fix the order of the derivation's sources in `derivation-hash'.

* guix/derivations.scm (derivation-hash): Sort SOURCES alphabetically as
  well.

Fix `imported-files' for files with a common prefix.

* guix/derivations.scm (imported-files)[builder]: For TAIL, check
  whether it exists before (mkdir TAIL).

* tests/derivations.scm ("imported-files"): Add file with a common
  prefix.

Add `add-to-store' test for flat files.

* tests/derivations.scm ("add-to-store, flat"): New test.

Augment `build-expression->derivation' with #:modules; add `http-fetch'.

* guix/derivations.scm (imported-modules): New procedure.
  (build-expression->derivation): New keyword argument `modules'.
  Use `imported-modules' when MODULES is non-empty, and pass it with
  `-L' to GUILE.

* guix/build/http.scm, guix/http.scm, tests/builders.scm: New files.

Add `imported-files'.

* guix/derivations.scm (imported-files): New procedure.
  (build-expression->derivation): Correctly handle inputs that are
  sources and not derivation paths.

* tests/derivations.scm ("imported-files"): New test.

Fix the `build-expression->derivation' fixed-output test.

* tests/derivations.scm ("build-expression->derivation"): Fix the
  builder to actually pass a bytevector to `put-bytevector'.

Add a `%current-system' fluid.

* guix/utils.scm (gnu-triplet->nix-system): New procedure.
  (%current-system): New variable.

* tests/utils.scm ("gnu-triplet->nix-system"): New test.

* tests/derivations.scm (%current-system): Remove.  Update users to
  use (%current-system) instead.

Add `add-to-store' with recursive directory storage.

* guix/store.scm (write-file): Implement directory recursive dump.
  (add-to-store): Fix the parameter list.

* tests/derivations.scm (directory-contents): New procedure.
  ("add-to-store, recursive"): New test.

tests: Don't hard-code the system type.

* tests/derivations.scm (%current-system): New variable.  Replace
  occurrences of "x86_64-linux" by %CURRENT-SYSTEM.

Add multiple-output support to `build-expression->derivation'.

* guix/derivations.scm (build-expression->derivation): Add `outputs'
  keyword parameter; pass it to `derivation'.  Define `%outputs' in the
  prologue.

* tests/derivations.scm ("build-expression->derivation with two
  outputs"): New test.

Add `derivation' test for multiple-output derivations.

* tests/derivations.scm ("multiple-output derivation"): New test.

Sort environment variables alphabetically in `derivation'.

* guix/derivations.scm (derivation)[env-vars-with-empty-outputs]: Sort
  the result alphabetically.

* tests/derivations.scm ("build derivation with 1 source"): Add
  environment variables.

Add support for fixed-output derivations in `build-expression->derivation'.

* guix/derivations.scm (build-expression->derivation): Pass HASH and
  HASH-ALGO to `derivation'.

* tests/derivations.scm ("build-expression->derivation for fixed-output
  derivation"): New test.

Add support for fixed-output derivations.

* guix/derivations.scm (read-derivation)[outputs->alist]: For
  fixed-outputs, convert HASH with `base16-string->bytevector'.
  (write-derivation): Likewise, convert HASH-ALGO to a string and HASH
  to a base16 string.
  (derivation-hash): Expect HASH to be a bytevector, not a string;
  convert HASH with `bytevector->base16-string'.

* tests/derivations.scm ("fixed-output derivation"): New test.

Add `base16-string->bytevector'.

* guix/utils.scm (base16-string->bytevector): New procedure.

* tests/utils.scm ("bytevector->base16-string->bytevector"): New test.

Add `base32-string->bytevector' and `nix-base32-string->bytevector'.

* guix/utils.scm (bytevector-quintet-set!,
  bytevector-quintet-set-right!, base32-string-unfold,
  base32-string-unfold-right, make-base32-string->bytevector,
  base32-string->bytevector, nix-base32-string->bytevector): New
  procedures.

* tests/utils.scm ("base32-string->bytevector",
  "nix-base32-string->bytevector"): New tests.

Augment `README'.

Add `build-expression->derivation'.

* guix/derivations.scm (%guile-for-build): New parameter.
  (build-expression->derivation): New procedure.

* tests/derivations.scm ("build-expression->derivation without inputs",
  "build-expression->derivation with one input"): New tests.

Allow derivations with input derivations.

* guix/derivations.scm (derivation-path->output-path): New procedure.
  (derivation-hash): Call `memoize'.  In the fixed-output case, convert
  HASH-ALGO to a string.  In the other case, sort inputs in the
  alphabetical order of their hex hash.  For inputs with no sub-drvs,
  add "out" as the sub-drv.

* guix/utils.scm (%nixpkgs-directory): New parameter.
  (nixpkgs-derivation, memoize): New procedures.

* tests/derivations.scm ("build derivation with 1 source"): Remove
  useless shebang.
  (%coreutils): New variable.
  ("build derivation with coreutils"): New test.