This lets record users choose an identifier other than 'this-record'.
* guix/records.scm (make-syntactic-constructor): Add #:this-identifier.
[wrap-field-value]: Honor it.
(define-record-type*): Add form with extra THIS-IDENTIFIER and honor
it.
* tests/records.scm ("define-record-type* & thunked & inherit & custom this"):
New test.