define-record-type*: Add the `thunked' field definition keyword.

* guix/utils.scm (define-record-type*)[make-syntactic-constructor]: Add
  a `thunked' parameter.
  (thunked-field?, field-bindings): New procedures.  Use the latter when
  generating `letrec*' bindings.
  [thunked-field?, thunked-field-accessor-name, field-spec->srfi-9,
  thunked-field-accessor-name]: New procedures.
  Use them when generating the `define-record-type' form, and to
  generated thunk field accessors, along call to
  `make-syntactic-constructor' with the new argument.
* tests/utils.scm ("define-record-type* & thunked",
  "define-record-type* & thunked & default",
  "define-record-type* & thunked & inherited"): New tests.