Bug: conj* and extend* for collections that aren't sequences

[countvajhula]

If a struct type implements gen:collection but not gen:sequence, then conj* and extend* result in a contract error.

(struct bag (contents)
  #:transparent
  #:methods gen:collection
  [(define (conj col elem)
     (bag (cons elem (bag-contents col))))])

main.rkt> (conj (bag (list 1 2 3)) 4)
(bag '(4 1 2 3))

main.rkt> (extend (bag (list 1 2 3)) (list 4))
(bag '(4 1 2 3))

main.rkt> (conj* (bag (list 1 2 3)) 4)
; conj*: broke its own contract
;   promised: sequence?
;   produced: (bag '(4 1 2 3))
;   in: the range of
;       (->* (collection?) #:rest any/c sequence?)
;   contract from: 
;       <pkgs>/collections-lib/data/collection/collection.rkt
;   blaming: <pkgs>/collections-lib/data/collection/collection.rkt
;    (assuming the contract is correct)
;   at: <pkgs>/collections-lib/data/collection/collection.rkt:44.3
; Context:
;  .../private/blame.rkt:347:0 body of "/Users/siddhartha/work/lisp/racket/generator-util/main.rkt"
;  .../contract/combinator.rkt:324:9
;  .../private/arrow-val-first.rkt:486:18
;  .../racket/repl.rkt:11:26

main.rkt> (extend* (bag (list 1 2 3)) (list 4))
; extend*: broke its own contract
;   promised: sequence?
;   produced: (bag '(4 1 2 3))
;   in: the range of
;       (->*
;        (collection?)
;        #:rest
;        (listof sequence?)
;        sequence?)
;   contract from: 
;       <pkgs>/collections-lib/data/collection/collection.rkt
;   blaming: <pkgs>/collections-lib/data/collection/collection.rkt
;    (assuming the contract is correct)
;   at: <pkgs>/collections-lib/data/collection/collection.rkt:43.3
; Context:
;  .../private/blame.rkt:347:0 body of "/Users/siddhartha/work/lisp/racket/generator-util/main.rkt"
;  .../contract/combinator.rkt:324:9
;  .../private/arrow-val-first.rkt:486:18
;  .../racket/repl.rkt:11:26