implemented marker procs for the GC resulting in huge speedups

todo.txt

@@ -10,9 +10,6 @@ version 0.9.0
   - Test capture of for loop vars; test generics;
   - test constant closures
 
-- GC: marker procs for native Nimrod GC and Boehm GC; precise stack marking;
-  escape analysis for string/seq seems to be easy to do too;
-  even further write barrier specialization
 - dead code elim for JS backend; 'of' operator for JS backend
 - const ptr/ref
 - unsigned ints and bignums; requires abstract integer literal type: 
@@ -133,6 +130,11 @@ Low priority
     need to recompile clients; er ... what about templates, macros or anything
     that has inlining semantics?
 - codegen should use "NIM_CAST" macro and respect aliasing rules for GCC
+- GC: precise stack marking;
+  escape analysis for string/seq seems to be easy to do too;
+  even further write barrier specialization
+- GC: marker procs Boehm GC
+- implement marker procs for assignment and message passing
 - warning for implicit openArray -> varargs conversion
 - implement explicit varargs; **but** ``len(varargs)`` problem remains! 
   --> solve by implicit conversion from varargs to openarray