(system base types) knows about variables

* module/system/base/types.scm (%tc7-variable): New tc7.
  (cell->object): Handle tc7-variable.

* test-suite/tests/types.test ("opaque objects"): Add a test.

module/system/base/types.scm

@@ -241,6 +241,7 @@ the matching bits, possibly with bitwise operations to extract it from BITS."
 ;; Cell types.
 (define %tc3-struct 1)
 (define %tc7-symbol 5)
+(define %tc7-variable 7)
 (define %tc7-vector 13)
 (define %tc7-wvect 15)
 (define %tc7-string 21)
@@ -424,6 +425,8 @@ using BACKEND."
            (match (cell->object buf backend)
              (($ <stringbuf> string)
               (string->symbol string))))
+          (((_ & #x7f = %tc7-variable) obj)
+           (inferior-object 'variable address))
           (((_ & #x7f = %tc7-string) buf start len)
            (match (cell->object buf backend)
              (($ <stringbuf> string)

test-suite/tests/types.test

@@ -101,6 +101,7 @@
    ((%make-void-port "w") port (? integer?))
    ((open-input-string "hello") port (? integer?))
    ((lambda () #t) program _)
+   ((make-variable 'foo) variable _)
    ((make-weak-vector 3 #t) weak-vector _)
    ((make-weak-key-hash-table) weak-table _)
    ((make-weak-value-hash-table) weak-table _)