* first implementation of the =trace and =dispose hooks for the cycle collector
* a cycle collector for ARC: progress
* manual: the .acyclic pragma is a thing once again
* gcbench: adaptations for --gc:arc
* enable valgrind tests for the strutils tests
* testament: better valgrind support
* ARC refactoring: growable jumpstacks
* ARC cycle detector: non-recursive algorithm
* moved and renamed core/ files back to system/
* refactoring: --gc:arc vs --gc:orc since 'orc' is even more experimental and we want to ship --gc:arc soonish
* cursors: first implementation
* added currently failing test
* .cursor works for doubly linked lists
* make -d:useMalloc work again
* added code to nil out refs in a destructor
* it's now called --gc:arc
* renderer.nim: render nkBreakState properly
* make simple closure iterators work without leaking
* mark user defined destructors with sfOverriden to simplify the logic
* refactoring in preparation to merge liftings and generic instantiations for destructors
* ast: introduce nkHiddenTryStmt for destructor generation in order to be able to optimize better the code later on
* renamed 'patterns' switch to 'trmacros' as it was totally misleading before
* destructors: introduce tfCheckedForDestructor flag in preparation of strict =destroy checking
* test for invalid/too late destructor introductions
* liftdestructors: make code robust for nimsuggest
* --newruntime works for hello world again
* newruntime: code generation for closures
