records: Add support for 'innate' fields.

* guix/records.scm (make-syntactic-constructor): Add #:innate parameter.
  [record-inheritance]: Honor it.
  [innate-field?]: New procedure.
  (define-record-type*)[innate-field?]: New procedure.
  Pass #:innate to 'make-syntactic-constructor'.
* tests/records.scm ("define-record-type* & inherit & innate",
  "define-record-type* & thunked & innate"): New tests.