fix ephemeron allocation

Only half(!) of the needed space was actually allocated. The extra
space is ony used after a GC, however, and a GC makes the extra room,
so that's why things haven't fallen over completely, but that's more
subtle than intended.

original commit: 3d72bc14b9247d6764809cb651403dbb4063a905

c/prim5.c

@@ -183,8 +183,11 @@ static ptr s_ephemeron_cons(car, cdr) ptr car, cdr; {
   ptr p;
 
   tc_mutex_acquire()
-  p = S_cons_in(space_ephemeron, 0, car, cdr);
+  find_room(space_ephemeron, 0, type_pair, size_ephemeron, p);
   tc_mutex_release()
+  INITCAR(p) = car;
+  INITCDR(p) = cdr;
+
   return p;
 }