threads clean up their heap

todo.txt

@@ -1,19 +1,9 @@
-* improve ``echo`` code generation for multi-threading
+* codegen for threadvars
 * two issues for thread local heaps:
   - must prevent to construct a data structure that contains memory 
     from different heaps:  n.next = otherHeapPtr
-  - must prevent that GC cleans up memory that other threads can still read...
-    this can be prevented if the shared heap is simply uncollected (at least
-    for now)
-
 
 * add --deadlock_prevention:on|off switch? timeout for locks?
-* make GC fully thread-safe; needs:
-  - thread must store its stack boundaries
-  - GC must traverse these stacks: Even better each thread traverses its
-    stack! No need to stop if you can help the GC ;-)
-  - isOnStack() needs to take them into account (SLOW?)
-  - GC must stop the world
   
 * implicit ref/ptr->var conversion; the compiler may store an object
   implicitly on the heap for write barrier efficiency! (Especially