records: Optimize 'recutils->alist' by avoiding regexps.

* guix/records.scm (%recutils-field-rx, %recutils-comment-rx,
  %recutils-plus-rx): Remove.
  (%recutils-field-charset): New variable.
  (recutils->alist): Adjust to use tests (string-ref line 0) instead of
  regexps.