define-record-type*: Add `letrec*' behavior.

* guix/utils.scm (define-record-type*)[make-syntactic-constructor]: Bind
  all the ((FIELD VALUE) ...) in a `letrec*'.  Adjust `field-value'
  accordingly.

* tests/utils.scm ("define-record-type* with letrec* behavior"): New
  test.