records: `alist->record' supports multiple-field occurrences.

* guix/records.scm (alist->record): Add `multiple-value-keys'
  parameter.  Update docstring, and honor it.
* tests/records.scm ("alist->record"): New record.

records: `recutils->alist' recognizes comments.

* guix/records.scm (%recutils-comment-rx): New variable.
  (recutils->alist): Match comments.
* tests/records.scm ("recutils->alist"): Add comments.

records: Add `recutils->alist' for public consumption.

* guix/records.scm (%recutils-field-rx): New variable.
  (recutils->alist): New procedure, formerly known as `fields->alist'.
* guix/scripts/substitute-binary.scm (fields->alist): Use it.
* tests/records.scm ("recutils->alist"): New test.

Move record utilities to (guix records).

* guix/utils.scm (define-record-type*): Move to...
* guix/records.scm: ... here.  New file.
* guix/build-system.scm, guix/packages.scm: Use it.
* guix/gnu-maintenance.scm: Likewise.
  (official-gnu-packages)[alist->record]: Remove.
* guix/scripts/substitute-binary.scm: Likewise.
  (alist->record, object->fields): Remove.
* tests/utils.scm ("define-record-type*", "define-record-type* with
  letrec* behavior", "define-record-type* & inherit",
  "define-record-type* & inherit & letrec* behavior",
  "define-record-type* & thunked", "define-record-type* & thunked &
  default", "define-record-type* & thunked & inherited"): Move to...
* tests/records.scm: ... here.  New file.