* cycle collector: new implementation
* cycle collector: make self-adaptive based on its previous effectiveness
* cycle collector: added Lins's jump stack to improve traversal from 3*N to 2*N
* cycle collector: make tests green
* API extensions and bugfixes
* code cleanup and use --gc:orc for tasyncawait
* allow defects to be caught even for --exceptions:goto (WIP)
* implemented the new --panics:on|off switch; refs https://github.com/nim-lang/RFCs/issues/180
* new implementation for integer overflow checking
* produce a warning if a user-defined exception type inherits from Exception directly
* applied Timothee's suggestions; improved the documentation and replace the term 'checked runtime check' by 'panic'
* fixes#13627
* don't inherit from Exception directly
