This implements "deterministic" exception handling for Nim based on goto instead of setjmp. This means raising an exception is much cheaper than in C++'s table based implementations. Supports hard realtime systems. Default for --gc:arc and the C target because it's generally a good idea and arc is all about deterministic behavior.
Note: This implies that fatal runtime traps are not catchable anymore! This needs to be documented.
* 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
