records: Add support for delayed fields.

records: Add support for delayed fields.

* guix/records.scm (make-syntactic-constructor): Add #:delayed
  parameter.
  [delayed-field?]: New procedure.
  [wrap-field-value]: Use it.
  (define-record-type*)[delayed-field?, wrapped-field?]: New procedures.
  [thunked-field-accessor-name]: Rename to...
  [wrapped-field-accessor-name]: ... this.
  [field-spec->srfi-9]: Change 'thunked' to 'wrapped'.
  [delayed-field-accessor-definition]: New procedure.
  Compute delayed-field accessors and emit them.  Pass #:delayed to
  'make-syntactic-constructor'.
* tests/records.scm ("define-record-type* & delayed",
  "define-record-type* & delayed & default",
  "define-record-type* & delayed & inherited"): New tests.