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.