Add `define-record-type*'.

* guix/utils.scm (define-record-type*): New macro.

* tests/utils.scm ("define-record-type*"): New test.

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 `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.

Fix `bytevector->nix-base32-string'.

* guix/utils.scm (bytevector-quintet-ref-right,
  bytevector-quintet-fold): New procedures.
  (bytevector-quintet-fold-right): Add `quintet-fold' parameter; use it
  instead of `bytevector-quintet-fold'.
  (bytevector->base32-string): Pass BYTEVECTOR-QUINTET-FOLD as the
  first parameter.
  (bytevector->nix-base32-string): Pass BYTEVECTOR-QUINTET-FOLD-RIGHT as
  the first parameter.

* tests/utils.scm ("sha256 & bytevector->nix-base32-string"): New test.

Add (guix utils), with `bytevector->base32-string'.

* guix/utils.scm, tests/utils.scm: New files.