add 'simple-root' for students to use in test suites that call gc:cons

pkgs/plai/gc2/private/gc-core.rkt

@@ -98,7 +98,13 @@
 (provide/contract
  [root? (-> any/c boolean?)]
  [root-name (-> root? any/c)]
- [make-root (-> any/c (-> location?) (-> location? void) root?)])
+ [make-root (-> any/c (-> location?) (-> location? void) root?)]
+ [simple-root (-> location? root?)])
+
+(define (simple-root n)
+  (make-root 'simple-root
+             (λ () n)
+             (λ (n2) (set! n n2))))
 
 (provide make-env-root)
 (define-syntax (make-env-root stx)

pkgs/plai/scribblings/collector2.scrbl

@@ -96,6 +96,10 @@ Determines if @racket[v] is a root.
   Updates @racket[root] to refer to @racket[loc].
 }
 
+@defproc[(simple-root [l location?]) root?]{
+  Makes a root that is initialized with @racket[l].                                            
+}
+
 @defproc[(make-root [name symbol?] [get (-> location?)] [set (-> location? void?)])
          root?]{
   Creates a new root. When @racket[read-root] is called, it invokes